Fix for listenToOnce mem. leak

This is a failing test demonstrating #3226, then a patch that makes the test pass.
I'd say compose covers that case better but w/e

On Mon, Oct 20, 2014 at 12:54 PM, James Smith <notifications@github.com>
wrote:

> In backbone.js:
>
> > @@ -239,6 +239,13 @@
> >        var id = obj._listenId || (obj._listenId = _.uniqueId('l'));
> >        listeningTo[id] = obj;
> >        if (!callback && typeof name === 'object') callback = this;
> > +      if (implementation === 'once') {
> > +        var cachedCallback = callback;
>
> I opted for _.wrap in the latest commit, as suggested by @jridgewell
> <https://github.com/jridgewell>.
>
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> <https://github.com/jashkenas/backbone/pull/3341/files#r19097218>.
>