Message matching in FFI should compare Message handle vs. string instead of two handles.

[ewalters-mitre]

const Mismatches *match_message(const Message *msg_1, const Message *msg_2); should instead have a signature of:
const Mismatches *match_message(const Message *msg_1, const char *actual_msg); since we're technically not matching a Message *, which is a Pact message handle against another handle. Instead we should be matching the Pact Message (with matching rules and expected message context) against an actual message as string content.

[alilleybrinker]

I recall not being sure how to pursue the right API design here. I'll need to investigate before I can estimate how tough this will be.

[ewalters-mitre]

I understand that this part was fuzzy. Happy to act as a sounding board if you need one.

If it looks like both params should be messages, then that's ok but it must be properly motivated.

[alilleybrinker]

After some spelunking in pact_matching and pact_verifier, I think we have an approach:

We'll add a new FFI constructor for Message called message_from_body that looks like message_from_body(const char *body, const char* content_type) -> *mut Message. The content_type field is necessary to initialize OptionalBody inside the Message, although in our case the type is always an XML content-type (application/xml). The Message returned from this new constructor can be used with match_message.