Fix multi part issue

[macisamuele]

The goal of this PR is to address an issue related to file uploads.
The issue was identified internally and ticketed on COREMOBAPI-1461

The issue could be summarised as: we're generating incorrect code in case of file parameters

Assuming a simple endpoint

"/send/file": {
    "post": {
        "consumes": ["multipart/form-data"],
        "parameters": [
            {
                "in": "formData",
                "type": "file",
                "name": "file",
                "required": true
            }
        ],
        "responses": {
            "200": {
                "description": "fafa"
            }
        }
    }
}

The generated code should look like

@JvmSuppressWildcards
interface DefaultApi {
  @retrofit2.http.Multipart    // <- different
  @Headers(
    "X-Operation-ID: "
  )
  @POST("/send/file")
  fun sendFilePost(
    @retrofit2.http.Part(    // <- different
      "file\"; filename=\"file") file:  RequestBody): Completable
}

while the following code is the currently generated content

@JvmSuppressWildcards
interface DefaultApi {
  @retrofit2.http.FormUrlEncoded
  @Headers(
    "X-Operation-ID: "
  )
  @POST("/send/file")
  fun sendFilePost(@retrofit2.http.Field("file\"; filename=\"file") file:  RequestBody): Completable
}

The issue is caused by the fact that swagger-codegen does not keep in consideration the operation level consumes key to properly set isMultipart attribute of Operation object.

[cortinico]

Other than the small change, looks fine to me.