only sort the headers by key not by value

--HG--
extra : rebase_source : 28c8db5c527838341f125e10f8ec770352c92b75
add test_preserve_header_value_order
- Fix some cases where the creation of extremely large output buffers (greater
  than 2GB, suspected to be buffers added via ``wsgi.file_wrapper``) might
  cause an OverflowError on Python 2.  See
  https://github.com/Pylons/waitress/issues/47.

See #47
restore py3 compat, coverage
- When the ``url_prefix`` adjustment starts with more than one slash, all
  slashes except one will be stripped from its beginning.  This differs from
  older behavior where more than one leading slash would be preserved in
  ``url_prefix``.

- If a client somehow manages to send an empty path, we no longer convert the
  empty path to a single slash in ``PATH_INFO``.  Instead, the path remains
  empty.  According to RFC 2616 section "5.1.2 Request-URI", the scenario of a
  client sending an empty path is actually not possible because the request URI
  portion cannot be empty.

- If the ``url_prefix`` adjustment matches the request path exactly, we now
  compute ``SCRIPT_NAME`` and ``PATH_INFO`` properly.  Previously, if the
  ``url_prefix`` was ``/foo`` and the path received from a client was ``/foo``,
  we would set *both* ``SCRIPT_NAME`` and ``PATH_INFO`` to ``/foo``.  This was
  incorrect.  Now in such a case we set ``PATH_INFO`` to the empty string and
  we set ``SCRIPT_NAME`` to ``/foo``.  Note that the change we made has no
  effect on paths that do not match the ``url_prefix`` exactly (such as
  ``/foo/bar``); these continue to operate as they did.  See
  https://github.com/Pylons/waitress/issues/46

Fixes #46
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.
- When waitress receives a ``Transfer-Encoding: chunked`` request, we no longer
  send the ``TRANSFER_ENCODING`` nor the ``HTTP_TRANSFER_ENCODING`` value to
  the application in the environment.  Instead, we pop this header.  Since we
  cope with chunked requests by buffering the data in the server, we also know
  when a chunked request has ended, and therefore we know the content length.
  We set the content-length header in the environment, such that applications
  effectively never know the original request was a T-E: chunked request; it
  will appear to them as if the request is a non-chunked request with an
  accurate content-length.

- Cope with the fact that the ``Transfer-Encoding`` value is case-insensitive.

Fixes #43
Merge remote-tracking branch 'upstream/master' into feature.shutdown-gracefuly