Support MS Exchange 2013 NTLM challenge

I have an MS Exchange 2013 server that likes to send this auth header as a challenge:

'www-authenticate': 'NTLM YadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYadaYada, Negotiate'

I'm not sure why MS chose to break the protocol, but anyway python-ntlm chokes on the superfluous "Negotiate" at the end, and removing it lets me communicate with the server.