Eventually support IncomingMessage/OutgoingMessage

[dougwilson]

Right now the signature is fresh(reqHeaders, resHeaders), which is of course nice for being agnostic, but slightly lame. I don't think we should remove this "agnostic-ness" per-se, but it would be nice to pass in IncomigMessage and OutgoingMessage objects so this lib can just handle weirdness like expressjs/express#2468

[dougwilson]

We can also move the statusCode and method checking or whatever in here as well if this happens, similar to type-is does with hasBody to make it easier to conform to HTTP specs.