fix the global keydown hook

The "keydown" event that this control listens for gets fired for other "dropdown" controls too. Honestly I'm not very sure how correctly those other controls use the attribute `data-toggle="dropdown"` in the example that I'm working on but in any case the fix that I made is good to have because it changes how the reference is saved as `$this.data('selectpicker')` instead of `$this.data('this')` which shall prevent clashes with other libraries and plugins.