Added support for arrays to `model.unset()`

I hope I kept the writing and test styles consistent.

This adds support for the `attr` argument in `model.unset()` to be an array.  It uses concat to ensure that attrs will always be an array and `_.object()` so the hash sent to `set` will be an array, since the second argument is an empty array all values are set to `undefined`.

The contributing guide said not to update the documentation so I left the comment above the function unchanged.

Avi