Fixes #649 and #1329 by making Session.headers a CaseInsensitiveDict,
and fixing the implementation of CID. Credit for the brilliant idea
to map `lowercased_key -> (cased_key, mapped_value)` goes to
@gazpachoking, thanks a bunch.
Changes from original implementation of CaseInsensitiveDict:
1. CID is rewritten as a subclass of `collections.MutableMapping`.
2. CID remembers the case of the last-set key, but `__setitem__`
and `__delitem__` will handle keys without respect to case.
3. CID returns the key case as remembered for the `keys`, `items`,
and `__iter__` methods.
4. Query operations (`__getitem__` and `__contains__`) are done in
a case-insensitive manner: `cid['foo']` and `cid['FOO']` will
return the same value.
5. The constructor as well as `update` and `__eq__` have undefined
behavior when given multiple keys that have the same `lower()`.
6. The new method `lower_items` is like `iteritems`, but keys are
all lowercased.
7. CID raises `KeyError` for `__getitem__` as normal dicts do. The
old implementation returned
6. The `__repr__` now makes it obvious that it's not a normal dict.
See PR #1333 for the discussions that lead up to this implementation