Add ability to document API operations that support multiple content type responses

Exploit an undocumented feature in Swagger 1.1 that allows multiple content-type responses to be tried out

- relevant 'supportedContentTypes' field will be renamed to 'produces' in 1.2 spec (release not imminent)
- default behaviour if no 'produces' specified is to return 'application/json' as this is current behaviour of swagger-ui