bpo-39548: Fix handling of 'WWW-Authenticate' header for Digest Auth (GH-18338)

bpo-39548: Fix handling of 'WWW-Authenticate' header for Digest Auth (GH-18338)

* bpo-39548: Fix handling of 'WWW-Authenticate' header for Digest authentication

 - The 'qop' value in the 'WWW-Authenticate' header is optional. The
   presence of 'qop' in the header should be checked before its value
   is parsed with 'split'.

Signed-off-by: Stephen Balousek <stephen@balousek.net>
* bpo-39548: Fix handling of 'WWW-Authenticate' header for Digest authentication

 - Add NEWS item

Signed-off-by: Stephen Balousek <stephen@balousek.net>
* Update Misc/NEWS.d/next/Library/2020-02-06-05-33-52.bpo-39548.DF4FFe.rst

Co-Authored-By: Brandt Bucher <brandtbucher@gmail.com>
Co-authored-by: Brandt Bucher <brandtbucher@gmail.com>
(cherry picked from commit 5e260e0fde211829fcb67060cfd602f4b679f802)

Co-authored-by: Stephen Balousek <sbalousek@users.noreply.github.com>