Fix for issue #538

Added check of WWW-Authenticate header for required security method
Added code to generate a Digest response for the Authentication header
Added utility code around the above.

Added computation of the Digest Authenticate header, and detection of whether it's Basic or Digest required by the server.

I don't currently save the nc between requests - I need a pointer on where/how I can do this for all service clients. (Can I just save this as a protected property internally to the ServiceClientBase class?)