* Added support for string responses
When a method/URL/response is defined to return string:
- If no content types are define, default to 'text/plain' instead of
'application/json'
- Add response handler, that returns the reponse string as-is if response
content-type is 'text/plain'
* Removed use of unused tag vendor tag
- The tag was vendorExtensions.x-codegen-response.isPrimitiveType
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