Operations with multiple request body content types

[karlvr]

We need to be able to differentiate when there are multiple request body content types. I think this is a generator's responsibility, but we can probably provide some default implementations...

One solution is to generate wrapper classes to differentiate the requests, for languages like Java where you could check the type of the request object. e.g. MyRequestTextHtml vs MyRequestTextPlain.

Another is to generate uniquely named operations, where each is responsible for a different request body content type. For languages like JavaScript.