Improve resilience to bad authorization headers

Catch IndexError when calling parse_keqv_list in
parse_authorization_header.  parse_keqv_list could raise an IndexError
exception if the header has an incomplete key=value pair.  Some examples
of this are a broken OAuth header (e.g., OAuth oauth_nonce=) and client
code that attempts to use IWA for authentication (i.e., Negotiate
b2F1dGhsaWI=).  IWA uses Base64 which may end with an "=".

Add unit tests for bad authorization headers raising ValueError.