schema cache: Do not reset ServerInfo dirty flag

Once dirty flag is set to True it must not be set back to False.
Otherwise changes are not written back to file.

https://fedorahosted.org/freeipa/ticket/6048

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

ipaclient/remote_plugins/__init__.py

@@ -59,7 +59,8 @@ class ServerInfo(collections.MutableMapping):
         return self._dict[key]
 
     def __setitem__(self, key, value):
-        self._dirty = key not in self._dict or self._dict[key] != value
+        if key not in self._dict or self._dict[key] != value:
+            self._dirty = True
         self._dict[key] = value
 
     def __delitem__(self, key):