Update comment.
Add Plusser type

Plusser type implements all extra http.response fields
this let's me wrap ResponseWriter multiple times.
Extract out WrapResponseWriter

theoretically, we should have a separate wrapper for each permutation of
Plusser interfaces, but this should cover the common case.
Add some test files.
Encapsulate WrapResponseWriter in middleware stack building

that way middleware writers don't have to worry about it.