[BUG] [java] Additional Properties unable to handle JSON arrays

Created by: ashatch

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
Have you tested with the latest master to confirm the issue still exists?
Have you searched for related issues/PRs?
What's the actual output vs expected output?

Description

pojo.moustache doesn't handle additionalProperties in the case where a property value is a JSON array. It fails with e.g:

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at path $[0]

The generated model object should support deserialization.

OpenAPI declaration file content or url

Example fragment from a specification:

components:
  schemas:
    SomeModel:
      type: object
      additionalProperties: true

Steps to reproduce

Using the above YAML specification, generate a java client, then test with an API that produces a property with an array value:

{
  "foo": ["value1"]
}

This yields a runtime failure:

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at path $[0]

Suggest a fix

I'm happy to raise a PR for a fix, which adds support for JSON arrays in pojo.moustache. The problem is the assumption that all properties that are not primitive are always objects:

} else { // non-primitive type
   instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));