AsyncHttpConsumer: Do not stop the consumer when exiting `self.handle` already

[matthiask]

Earlier self.handle always disconnected the HTTP client and stopped the consumer when exiting. This meant that the consumer couldn't receive any messages from the channel layer at all, making it impossible to send e.g. messages from workers to connected HTTP clients via long polling or server-sent events.

[matthiask]

There were reasons (tm) for the addition of the StopConsumer and self.disconnect lines, but I'm not sure I understand them after rereading the pulls. Maybe now we're back to the situation where a dying consumer does not disconnect the HTTP connection?

Also, it's a bit unexpected that tests pass. I mean it's nice obviously, but I also suspect that coverage is ... not very good in this particular area.