sasl_sspi: Allow DIGEST-MD5 to use current windows credentials

Fixed the ability to use the current log-in credentials with DIGEST-MD5.
I had previously disabled this functionality in commit 607883f1 as I
couldn't get this to work under Windows 8, however, from testing HTTP
Digest authentication through Windows SSPI and then further testing of
this code I have found it works in Windows 7.

Some further investigation is required to see what the differences are
between Windows 7 and 8, but for now enable this functionality as the
code will return an error when AcquireCredentialsHandle() fails.