Prevent infinite recursion when server responds with 401 to an authentication attempt.

I found that requests-kerberos recurses until it blows the stack in the case where:

1. Server responds with 401 Unauthorized.
2. requests-kerberos retries the request with an Authorization header.
3. Server responds with 401 again, because the authenticated user has not been granted access.

This pull request changes the behaviour such that the response to `handle_401()` is returned with no further processing.

Tested with SharePoint Server 2013.