use concat, not union to add callback

`_.union` will make each item in the resulting array unique, which I don't think is the intended result. For example, if `fn` was `setWindowSize` which was called with equivalent width/height values, you'd have a problem (one of them would be stripped out of `args`).

I'm not entirely sure what the intention is here and if concat is sufficient, but it seems like union is definitely not what is desired here.