Add 'xfail' function from py.test
Fix caching_property

We set the cache value in the class's __dict__ to be the same name as
the property.  This broke things, so the __dict__ entry is now prefixed
with "_cache_".
Miscellaneous test fixes, add Morsel equality comparison
Add CallbackMultiDict and tests
Add implementation for WSGIResponseCookiesMixin, and tests
Allow indexing cookies with either bytes or str
Simplify the serialization code for max-age in cookies