Fix bug in handling multiple WWW-Authenticate headers
Fix bug in handling multiple WWW-Authenticate headers

When the server offered multiple authentication methods:

WWW-Authenticate: Negotiate
WWW-Authenticate: Basic realm="My Realm"

requests-negotiate failed to parse the resulting header.  This patch fixes
the issue, and also uses named matches to make the logic more clear.
slightly easier to read and also rfc7235 compliant.

ox-it/requests-negotiate