py3: base64 encoding/decoding returns always bytes don't mix it

Using unicode(bytes) call causes undesired side effect that is inserting
`b` character to result. This obviously causes issues with binary base64 data

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

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

ipaserver/plugins/baseldap.py

@@ -1036,7 +1036,7 @@ last, after all sets and adds."""),
                     except ValueError:
                         if isinstance(delval, bytes):
                             # This is a Binary value, base64 encode it
-                            delval = unicode(base64.b64encode(delval))
+                            delval = base64.b64encode(delval).decode('ascii')
                         raise errors.AttrValueNotFound(attr=attr, value=delval)
 
         # normalize all values