Add QUIT to the list of commands acceptable for PUBSUB channels

[robhague]

I have a long-running Tornado server where I'd like to have a number of relatively short-lived Redis PUBSUB connections (one per HTTP client connection). To avoid this set of Redis connections growing indefinitely, I want to close them once the HTTP client disconnects. However, Toredis currently asserts that QUIT cannot (as a normal command) be run on PUBSUB connections.

This change adds QUIT to the whitelist of commands for PUBSUB connections. I've added a unit test for the behaviour (which fails as expected without the change to the whitelist), and it appears to work as desired in my application. I can't see anything in the Redis documentation that says QUIT can't be used on PUBSUB channels; however, if I'm missing some subtlety let me know and I'll investigate further.