Use an interface for ContentTypeFromMessage

[wtlangford]

This allows custom Marshaler implementations to vary the HTTP response's Content-Type based on the returned proto.

Because of the comments around adding the interface param to the existing ContentType() function, I left the interface unexported, so that it doesn't become part of the public API of the package.

Alternatively, it could be exported and used to eventually deprecate the old Marshaler interface in favor of a more flexible ContentTypeFromMessage.

// A ContentTypeMarshaler is a Marshaler that can vary its Content-Type based on the response value
type ContentTypeMarshaler interface {
   Marshaler
   ContentTypeFromMessage(v interface{}) string
}

Is there a preference between the two approaches?

[johanbrandhorst]

Hi William, thanks for the PR! Do you mind explaining why you want this feature added? I appreciate that this is a pretty minimally disruptive change, so I don't have any reservations, but if you have a particular use case in mind it would help me understand why the existing solution isn't good enough.

[wtlangford]

We've added a custom marshaler in our application that wraps the existing HTTPBodyMarshaler and provides some additional functionality (mapping the request body into an google.api.HttpBody). Unfortunately, when we use that marshaler, we no longer have the Content-Type header properly set on responses that use google.api.HttpBody.

I'm happy to look at separately contributing back the request-mapping functionality, but this felt like a useful way to allow other custom marshalers to use ContentTypeFromMessage.

[johanbrandhorst]

Hi, please rebase on master, I've moved some folders around, but otherwise SGTM.