- The canonical source for :method, :scheme and :path are the .method, .scheme
and .path attributes on the request object.
- These pseudo-headers are stripped after reading the request, and re-inserted
just before sending.
- The :authority header remains, and should be handled analagously to the Host
header in HTTP1 with respect to display and user interaction.
This commit introduces MultiDict, a multi-dictionary similar to
ODict, but with improved semantics (as in the Headers class).
MultiDict fixes a few issues that were present in the Request/Response
API. In particular, `request.cookies["foo"] = "bar"` has previously been a
no-op, as the cookies property returned a mutable _copy_ of the cookies.
