Allow user to override content type

[mefellows]

I'm writing a test for a Twirp client using this package, and have noticed it doesn't add the ;message=WhateverIAmSending parameter on the content type.

This results in the following failure:

  Mock server failed with the following mismatches:

	0) The following request was incorrect:

            	POST /twirp/twirp.example.haberdasher.Haberdasher/FindHat

			 1.0 Expected body with content type application/protobuf;message=FindHatRPC but was application/protobuf

In this case, the client is not configurable to override this, and it's not clear (at least to me) if this is the standard Twirp behaviour (perhaps it's a gRPC specific thing?), and so it seems it's not safe to assume this content-type.

[rholshausen]

Without the message=WhateverIAmSending attribute, there is no way to know which message from the proto descriptors to use (there could be multiple interactions in the Pact file for different messages).

[mefellows]

Yeah, I can see the challenge. This does seem to require clients to change their behaviour though to suit the test framework (in this case, requiring them to add content-type attributes that aren't there by default), which in general I'm not a fan of.

Given that we know the current interaction(s) being tested at that moment, why must we encode that information into the content-type header and not just compare it to the expected protobuf definition provided in the test?