Correctly decode certificates

[tehwalris]

FreeIPA sends certificates in a special way. In the schema they are either string or Certificate. In the response they are sent as {"__base64__": "thecertinbase64"}. The current implementation either crashes or returns an interface{}. It should not crash and should return string or byte[].

Yes, there is a reason. Certificates are actually modeled as maps with a `__base64__` subfield, so casting them as strings failed.

Originally posted by @raphink in #7