[reSIProcate] A couple more minor issues from torture-tests

[Byron Campen <bcampen@xxxxxxxxxxxx>]

1. Currently ServerRegistration succeeds when the aor has a scheme  
other than sip or sips. This is a pretty easy fix (in  
ServerRegistration::dispatch, we check the scheme, and send a 400 if  
it isn't correct).

2. Currently, the authentication functions in Helper do not check  
whether the authentication scheme is "Digest" (they assume that  
"Digest" is being used without verifying whether this is true). This  
is also easily fixed.

Unless anyone has objections, I will be applying fixes for these to  
the main trunk.

Best regards,
Byron Campen

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