using notify listener with all-caps channel names

Hello,

I have a project where we set all our notify listeners up using all-caps channel names. In applying your package I found this didn't work, since the add_listener function sends the channel to postgres as a quoted identifier. I monkey patched as such to get it to work for my project:

```
async def add_listener(self, channel, callback, quote=True):
"""Add a listener for Postgres notifications.

:param str channel: Channel to listen on.

:param callable callback:
A callable receiving the following arguments:
**connection**: a Connection the callback is registered with;
**pid**: PID of the Postgres server that sent the notification;
**channel**: name of the channel the notification was sent to;
**payload**: the payload.

:param quote:
specifies the channel name should should be sent as quoted identifier
"""
self._check_open()
if not quote:
channel = channel.lower()
if channel not in self._listeners:
if quote:
await self.fetch('LISTEN {}'.format(utils._quote_ident(channel)))
else:
await self.fetch('LISTEN {}'.format(channel))
self._listeners[channel] = set()
self._listeners[channel].add(callback)

```

Would you consider a pull request with this change? I would think I am not the only one who uses the listeners this way.

Thanks!

Search for add_listener and utils._quote_ident, then reproduce the reported behavior with an all-caps PostgreSQL notification channel. Done means all-caps channels can be registered and receive notifications while existing listener behavior remains intact.

Written by the indexing model from the issue text.