Switching order of service and host for digest-uri

The order of service and host was inverted. Just changing that I could use it for my XMPP implementation.

Here (line 76) you can also see how it's implemented in twisted: http://twistedmatrix.com/trac/browser/tags/releases/twisted-8.2.0/twisted/words/protocols/jabber/sasl_mechanisms.py#L76