Verify compatibility with RFC2617 Digest Access Authentication

Check Digest authentication mechanism implementation for incompatibilities with the RFC https://datatracker.ietf.org/doc/html/rfc2617#section-1.2.

It is mentioned that authentication scheme should be case insensitive so clients should be able to provide authorization header begginning with "Digest " prefix in a case insensitive manner. In our implementation of Digest authentication mechanism this is not currently the case.