Allow passing Request and Response objects

[palanik]

Use headers from http.IncomingMessage / http.OutgoingMessage, while providing backward compatibility.

Also including test cases.

[dougwilson]

👎 adding a dependency on the http module for such a trivial thing someone can do in their code is not great; that is a large dependency that can get added in a browserify bundle, for example. If we are going to accept those objects, at least we should only use the public APIs, not private APIs.