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.
9c873d63f4