py3: ipaldap: properly encode DNSName to bytes

The encode method of LDAPClient didn't return DNSName as bytes but
string in py3. In py2 it returns non-unicode string so it can be encoded
safely by ascii as to_text() method returns only ascii characters.

https://fedorahosted.org/freeipa/ticket/4985

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

ipapython/ipaldap.py

@@ -849,7 +849,7 @@ class LDAPClient(object):
                               Principal)):
             return six.text_type(val).encode('utf-8')
         elif isinstance(val, DNSName):
-            return val.to_text()
+            return val.to_text().encode('ascii')
         elif isinstance(val, bytes):
             return val
         elif isinstance(val, list):