support google.api.HttpBody

[theRealWardo]

I'd like my gateway to understand that I'm returning a google.api.HttpBody proto that should be the actual HTTP body of the response.

this ended up not being so easy because of the way the Content-Type handling is being done now. In order to do this properly, the following diff is necessary:

-       // ContentType returns the Content-Type which this marshaler is responsible for.
-       ContentType() string
+       // ContentType returns the response Content-Type for "v".
+       ContentType(v interface{}) string

further, in order to do this I'd propose a new Marshaler that wraps the current JSONPb Marshaler. the new Marshaler would be the new default as well as I believe most users want this behavior. does this seem reasonable?

[achew22]

I'm going to consolidate this into just the PR #458