[BUG][java] CustomTypeAdapterFactory incorrectly assumes non-primitives are json objects

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?
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Description

pojo.moustache's CustomTypeAdapterFactory.create populates properties by testing for e.g. primitive types for values. It's else block makes an assumption that if it isn't a primitive type, it must be an object:

gson.toJsonTree(entry.getValue()).getAsJsonObject()

This fails when considering arrays.

openapi-generator version

master

Suggest a fix

I'll raise a PR.