Fix Elm generator for polymorphism

[eriktim]

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\. Apparently something is broken, the pet's category is named object instead, see Models are now typed as Object when properties of another Model #79. This PR does not alter the examples any further.
• Filed the PR against the correct branch: Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

The support for polymorphism/discriminators was accidentally dropped by adding support for array schemas. This PR fixes correctly re-adds the array schemas support.

[wing328]

I merged this into master locally and updated the Elm samples. The test result looks good:

elm|trenneman-fix-elm ⇒ mvn integration-test
[INFO] Scanning for projects...
[INFO]                                                                         
[INFO] ------------------------------------------------------------------------
[INFO] Building Elm Petstore Client 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO] 
[INFO] --- maven-dependency-plugin:2.8:copy-dependencies (default) @ ElmClientTests ---
[INFO] 
[INFO] --- exec-maven-plugin:1.2.1:exec (bundle-test) @ ElmClientTests ---
Success! Compiled 1 module.
Successfully generated index.html
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 3.113 s
[INFO] Finished at: 2018-05-17T19:03:42+08:00
[INFO] Final Memory: 11M/245M
[INFO] ------------------------------------------------------------------------

I'll merge this into master and update the Elm client with a separate PR.