Fix for listenToOnce mem. leak

This is a failing test demonstrating #3226, then a patch that makes the test pass.
As a follow-up: `eventsAPI` would be the place to pull out hash maps, but [its call signature doesn't work](https://github.com/jashkenas/backbone/blob/1.1.2/backbone.js#L192) with `listenTo`/`listenToOnce`; it naïvely assumes that the event name is going to be the method's first parameter, which works with `on`/`off`/`once`/`trigger`, but isn't the case with the `listenTo`s.