[Apex] better handling of different content-type in the response

Created by: henryh-force

openapi-generator/modules/openapi-generator/src/main/resources/apex/OAS.cls

If you send a response that contains anything in addition to "application/json" nothing is returned.

Set the toReturnValue to validate for more than just application/json e.g. UTF-8 within the Content Type.

To fix #7500 (closed)