[Scala] Fix async helper methods when body is optional

[gmarz]

Closes #7272

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Description of the PR

Closes #7272

[jimschubert]

This change is most likely fine.

I wanted to comment on the "body is optional" comment, though.

I believe None would get serialized as null here, and not an empty body. The "required" property of a parameter for Swagger/OpenAPI 2.0 specs says:

Determines whether this parameter is mandatory. If the parameter is in "path", this property is required and its value MUST be true. Otherwise, the property MAY be included and its default value is false.

It's been a few years since I've used json4s, but I believe the way to truly support required=false for body parameters per swagger spec would be to implement a custom serializer per type and only write when the Option value is defined.

This isn't a deficiency in your PR, it's a confusion of the 2.0 specification, in which required refers to the json structure and not the data within the structure. This is fixed in Open API 3.0.1 for schema objects using the nullable attribute (see https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.1.md#schemaNullable).

[gmarz]

I believe None would get serialized as null here, and not an empty body.

I might be wrong (haven't actually tested this yet), but it looks like the async methods will write an empty body.

[wing328]

CircleCI reported the following errors:

[ERROR] /home/ubuntu/swagger-codegen/samples/client/petstore/scala/src/main/scala/io/swagger/client/api/PetApi.scala:56: error: not found: type JValue
[ERROR]   implicit val jvalueReader: ClientResponseReader[JValue] = ClientResponseReaders.JValueReader
[ERROR]                                                   ^
[ERROR] /home/ubuntu/swagger-codegen/samples/client/petstore/scala/src/main/scala/io/swagger/client/api/StoreApi.scala:54: error: not found: type JValue
[ERROR]   implicit val jvalueReader: ClientResponseReader[JValue] = ClientResponseReaders.JValueReader
[ERROR]                                                   ^
[ERROR] /home/ubuntu/swagger-codegen/samples/client/petstore/scala/src/main/scala/io/swagger/client/api/UserApi.scala:54: error: not found: type JValue
[ERROR]   implicit val jvalueReader: ClientResponseReader[JValue] = ClientResponseReaders.JValueReader
[ERROR]                                                   ^
[ERROR] three errors found
[ERROR] Failed to execute goal net.alchim31.maven:scala-maven-plugin:3.1.5:compile (scala-compile-first) on project swagger-scala-client: wrap: org.apache.commons.exec.ExecuteException: Process exited with an error: 1 (Exit value: 1) -> [Help 1]

I wonder if you can take a look when you've time.

UPDATE: I merged with master locally and no longer can repeat the error. I've pushed the branch to CircleCI to verify again. Let's see how it goes.

[wing328]

CircleCI tests passed via https://circleci.com/gh/swagger-api/swagger-codegen/3321