cert-find: fix 'issuer' option

The 'issuer' option of cert-find was recently changed from Str to
DNParam, however, 'ra.find' expects a string and throws when it
receives a DN.

When constructing the dict that gets passed to 'ra.find', turn
DNParams into strings.

Part of: https://fedorahosted.org/freeipa/ticket/5381

Reviewed-By: Jan Cholasta <jcholast@redhat.com>

ipaserver/plugins/cert.py

@@ -1031,6 +1031,8 @@ class cert_find(Search, CertMethod):
                     continue
                 if isinstance(value, datetime.datetime):
                     value = value.strftime(PKIDATE_FORMAT)
+                elif isinstance(value, DN):
+                    value = unicode(value)
                 ra_options[name] = value
             if sizelimit is not None:
                 if sizelimit != 0: