prevent to create new Revocation with empty serial when revokeCertificate is called with crt_id

caucase/storage.py

@@ -347,7 +347,7 @@ class Storage(object):
               " since %r day(s)." % (serial, -1*expire_in.days))
 
     revoke = Revocation(
-        serial=serial,
+        serial=cert.serial,
         creation_date=datetime.utcnow(),
         reason=reason,
         crt_expire_after=cert.expire_after