Follows up on the fix for ELY-2576.

For ELY-2576, DigestAuthenticationMechanism#getH_A1 was updated because it was previously using digest-md5 in all cases for the credential algorithm.

We switched to using getMechanismName() instead. This works for the DIGEST-SHA-256 and DIGEST-SHA-512-256 mechanism names but not for DIGEST. For DIGEST, digest-md5 needs to be used as before.