Add "produces" tag to control the accept headers.

This fixes the new API not working with "Accept: application/json", but
instead needing "Accept: application/json; charset=UTF-8"
Add the ability to hide the response content type Swagger field.

I'm not sure this is the best way to be doing this, but this dropdown
becomes rather duplicative/confusing if you're preferring a path-based
parameter for picking the response format (eg, the file extension
approach of ".json" or ".xml").