Fix multi_exec for reconnecting_redis

Had some troubles with multi-exec. This ain't gonna work, just to show you changes.
to keep it a simple generator you can get and pass `conn` as argument to `_send_pipeline` where it called in `_do_execute`:
```python
@asyncio.coroutine
     def _do_execute(self, *, return_exceptions=False):
         # get alive connection in case of AutoConnector
         yield from asyncio.gather(*self._send_pipeline(conn),
                                   loop=self._loop,
                                   return_exceptions=True)
```