[Swift4] Fix default value for enums (#652)

[grEvenX]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

This fixes an issue that makes the swift 4 api client fail to compile when using enums with a default value.
The fix changes the toDefaultValue method to check if the property is of enum type, and then returns a properly formatted value.
Looking into the code, I wonder why toEnumDefaultValue is not being used here, which seems to be the case for the java implementation. See issue details in #652 for example and expected output.

CC: @jgavris @ehyche @Edubits @jaz-ah @d-date

[grEvenX]

Haven't run the scripts: ./bin/swift4-petstore.sh like mentioned in the checklist since I saw that it hasn't been run in a good while (so it caused more changes than this PR). Should probably run though :)

[jaz-ah]

+1 LGTM

[jaz-ah]

(maybe we just run the petstore script in a followup pr which only has that change in it to catch up)

[wing328]

I've update the Swift4 petstore samples and tests ran successfully:

▸ Test execute Succeeded
All tests
Test Suite SwaggerClientTests.xctest started
PetAPITests
    ✓ test1CreatePet (1.097 seconds)
    ✓ test2GetPet (0.618 seconds)
    ✓ test3DeletePet (0.585 seconds)
StoreAPITests
    ✓ test1PlaceOrder (0.768 seconds)
    ✓ test2GetOrder (0.572 seconds)
    ✓ test3DeleteOrder (0.707 seconds)
    ✓ testDownloadProgress (0.616 seconds)
UserAPITests
    ✓ testLogin (0.496 seconds)
    ✓ testLogout (0.526 seconds)
    ✓ testPathParamsAreEscaped (0.000 seconds)


Executed 10 tests, with 0 failures (0 unexpected) in 5.986 (5.990) seconds

git diff is only showing the following changes:

diff --git a/samples/client/petstore/swift4/default/PetstoreClient/Classes/OpenAPIs/APIs/FakeAPI.swift b/samples/client/petstore/swift4/default/PetstoreClient/Classes/OpenAPIs/APIs/FakeAPI.swift
index 00f689db5..7dc52cc3b 100644
--- a/samples/client/petstore/swift4/default/PetstoreClient/Classes/OpenAPIs/APIs/FakeAPI.swift
+++ b/samples/client/petstore/swift4/default/PetstoreClient/Classes/OpenAPIs/APIs/FakeAPI.swift
@@ -389,13 +389,13 @@ open class FakeAPI {
      To test enum parameters

      - parameter enumHeaderStringArray: (header) Header parameter enum test (string array) (optional)
-     - parameter enumHeaderString: (header) Header parameter enum test (string) (optional, default to "-efg")
+     - parameter enumHeaderString: (header) Header parameter enum test (string) (optional, default to .-efg)
      - parameter enumQueryStringArray: (query) Query parameter enum test (string array) (optional)
-     - parameter enumQueryString: (query) Query parameter enum test (string) (optional, default to "-efg")
+     - parameter enumQueryString: (query) Query parameter enum test (string) (optional, default to .-efg)
      - parameter enumQueryInteger: (query) Query parameter enum test (double) (optional)
      - parameter enumQueryDouble: (query) Query parameter enum test (double) (optional)
-     - parameter enumFormStringArray: (form) Form parameter enum test (string array) (optional, default to "$")
-     - parameter enumFormString: (form) Form parameter enum test (string) (optional, default to "-efg")
+     - parameter enumFormStringArray: (form) Form parameter enum test (string array) (optional, default to .$)
+     - parameter enumFormString: (form) Form parameter enum test (string) (optional, default to .-efg)
      - parameter completion: completion handler to receive the data and the error objects
      */
     open class func testEnumParameters(enumHeaderStringArray: [String]? = nil, enumHeaderString: EnumHeaderString_testEnumParameters? = nil, enumQueryStringArray: [String]? = nil, enumQueryString: EnumQueryString_testEnumParameters? = nil, enumQueryInteger: EnumQueryInteger_testEnumParameters? = nil, enumQueryDouble: EnumQueryDouble_testEnumParameters? = nil, enumFormStringArray: [String]? = nil, enumFormString: EnumFormString_testEnumParameters? = nil, completion: @escaping ((_ data: Void?,_ error: Error?) -> Void)) {
@@ -414,13 +414,13 @@ open class FakeAPI {
      - GET /fake
      - To test enum parameters
      - parameter enumHeaderStringArray: (header) Header parameter enum test (string array) (optional)
-     - parameter enumHeaderString: (header) Header parameter enum test (string) (optional, default to "-efg")
+     - parameter enumHeaderString: (header) Header parameter enum test (string) (optional, default to .-efg)
      - parameter enumQueryStringArray: (query) Query parameter enum test (string array) (optional)
-     - parameter enumQueryString: (query) Query parameter enum test (string) (optional, default to "-efg")
+     - parameter enumQueryString: (query) Query parameter enum test (string) (optional, default to .-efg)
      - parameter enumQueryInteger: (query) Query parameter enum test (double) (optional)
      - parameter enumQueryDouble: (query) Query parameter enum test (double) (optional)
-     - parameter enumFormStringArray: (form) Form parameter enum test (string array) (optional, default to "$")
-     - parameter enumFormString: (form) Form parameter enum test (string) (optional, default to "-efg")
+     - parameter enumFormStringArray: (form) Form parameter enum test (string array) (optional, default to .$)
+     - parameter enumFormString: (form) Form parameter enum test (string) (optional, default to .-efg)
      - returns: RequestBuilder<Void>
      */
     open class func testEnumParametersWithRequestBuilder(enumHeaderStringArray: [String]? = nil, enumHeaderString: EnumHeaderString_testEnumParameters? = nil, enumQueryStringArray: [String]? = nil, enumQueryString: EnumQueryString_testEnumParameters? = nil, enumQueryInteger: EnumQueryInteger_testEnumParameters? = nil, enumQueryDouble: EnumQueryDouble_testEnumParameters? = nil, enumFormStringArray: [String]? = nil, enumFormString: EnumFormString_testEnumParameters? = nil) -> RequestBuilder<Void> {

If no further question/feedback. I'll merge it over the weekend.

(@grEvenX my apologies for the delay in reviewing this PR)

[wing328]

@grEvenX thanks again for the PR, which is included in the v3.3.0 minor release: https://twitter.com/oas_generator/status/1046941449609068544