This commit improves Python 3 compatibility and fixes two multidict
issues:
1. Headers.items(multi=True) now decodes fields
2. MultiDict.clear(item) has been removed, as Python's
MutableMapping already defines .clear() with different
semantics. This is confusing for everyone who expects a
dict-like object. `.pop("attr", None)` is not fantastic,
but it's the Python way to do it.
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.
