Extend issued-by? to support X509CRLHolder

When you deserialize a CRL as created by generate-crl, you don't get
the same CRL class back - you get a BouncyCastle X509CRLHolder.
This commit adds support for this class to the issued-by? multimethod.
I'm saying that they wouldn't even go into the testutils jar.  They could be test utility functions in the same namespace as the actual tests (in other projects) that were using them.  Sounds like I may be getting outvoted again though.