Swift4 -- some msg classes being encoded with Optional(value)

Description

Our GCP Endpoints V2 API was failing with 400 errors before ever reaching our server code. I used Postman to validate the API was working properly. I then moved to the client and began debugging the generated Swagger lib ... specifically
let parameters = JSONEncodingHelper.encodingParameters(forEncodableObject: body)

and discovered that the value in key "jsonData" (when decoded), looked like this:
{ "FieldName": "Optional(2)"}
which of course is not valid JSON.

Swagger-codegen version

Latest Swift4 as of 8/1/2018

Command line used for generation

java -jar /Users/dgaedcke/dev/swagger-codegen/swagger-codegen-cli.jar generate -c apigen_config.json -l swift4 -i /Users/dgaedcke/dev/tsSwagger/tstone1_0_1openapi.json -o /Users/dgaedcke/Downloads/TsApiBackend

Steps to reproduce

I'm working on a simple case & will post it later

Suggest a fix/enhancement

my 1st thought was that the Swift compiler was not auto-synthesizing "Codable" for some of the generated Msg Structs and thus it was using the "backup" version of this method:

open class func encodingParameters<T:Encodable>(forEncodableObject encodableObj: T?)

but reviewing the generated code, I see that all ARE marked Codable so that's clearly not the cause. I'm still investigating.

Start at JSONEncodingHelper.encodingParameters(forEncodableObject:) and compare the generic Encodable path with the backup overload used by the generated Swift4 message structs. Reproduce the reported GCP Endpoints request once a minimal case is available; done means encoded JSON contains values such as 2 rather than strings like "Optional(2)".