Passing is_master flag through on reset

 - is_master is not passed through when recreating the ConnectionPool.
 - This was causing unexpected behaviour where we would specifiy that we
   wish to connect to the slave, but the master would be read from
   instead.

redis/sentinel.py

@@ -129,6 +129,7 @@ def _checkpid(self):
             self.disconnect()
             self.reset()
             self.__init__(self.service_name, self.sentinel_manager,
+                          is_master=self.is_master,
                           connection_class=self.connection_class,
                           max_connections=self.max_connections,
                           **self.connection_kwargs)