ipalib.rpc: Send base64-encoded data as string under Python 3
    
    Python 3's JSON library cannot deal with bytes, so decode
    base64-encoded data to string.
    
    Part of the work for https://fedorahosted.org/freeipa/ticket/4985
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Petr Spacek <pspacek@redhat.com>