Allow deselecting when multiple=false

[fizz]

I found myself turning multiple on in order to allow deselecting after making a selection, and then I realized it was kind of silly. You should be able to uncheck by clicking the same option again. I can fake it by having an item whose value is null, but it just doesn't feel right to ask people to click on a fake option just to change their minds.

[sukima]

This is an interesting idea. Originally this was meant to be a drop in replacement to the native select which does not uncheck like you describe. Many libraries (including Ember) offered a null element to accomplish this in the native select.

However, considering Ember 2.0 got rid of it's native select and left things up to the user, maybe it is worth doing things differently in this project.