Superuser specification: only overwrite values if specified.

patroni/postgresql.py

@@ -131,7 +131,7 @@ class Postgresql:
     def connection(self):
         if not self._connection or self._connection.closed != 0:
             r = parseurl('postgres://{}/postgres'.format(self.local_address))
-            r['user'] = self.superuser['user']
+            r.update(self.superuser)
             self._connection = psycopg2.connect(**r)
             self._connection.autocommit = True
             self.server_version = self._connection.server_version