only sort the headers by key not by value

--HG--
extra : rebase_source : 28c8db5c527838341f125e10f8ec770352c92b75
add test_preserve_header_value_order
Merge branch 'master' of github.com:domruf/waitress into domruf-master
Add a change note, fix test so it passes on Python versions without
assertIn, add an NB: to the code.