Crash on unsubscribing from a nil channel

[soffes]

I can reproduce this every time with the nightly using the following code:

[self.pusherClient unsubscribeFromChannel:nil];

The workaround is to check for nil before unsubscribing.

[soffes]

Per our conversation on Twitter, it should throw an exception or assert if this is nil.

[lukeredpath]

I'm strongly in favour of enforcing the contract of not passing nil to this method, so I'll likely add an assertion to assist debugging.

[soffes]

Sounds great. You won me over :)