Pass the Model object to the callback function instead of the row array

[ChrisReid]

This does not break old code because the Model object implements ArrayableInterface. We can still access the callback with

function($m) { return $m['id']; }

as well as

function($m) { return $m->id; }

Now though, we can access modified attributes from Eloquent getters as well as relations on each model. A nice easy change without breaking backwards compatibility. :-)