Issue #2211: Updated the implementation of the http.cookies.Morsel class.

Setting attributes key, value and coded_value directly now is deprecated.
update() and setdefault() now transform and check keys.  Comparing for
equality now takes into account attributes key, value and coded_value.
copy() now returns a Morsel, not a dict.  repr() now contains all attributes.
Optimized checking keys and quoting values.  Added new tests.
Original patch by Demian Brecht.