[5.3] add $default parameter to query builder when() method

[tomschlick]

Trying this again (old PRS #15422 #15421)

This allows you to specify a default closure to run if the when() $value attribute is false.

I recently found myself creating a sorting / filtering system with defaults that the user could change. Without this you would have to do all kinds of wackiness to clear the previous where() clause on $value being true.

Here is an example of this in action

Time::when($request->has('type'), function ($query) use ($request) {
    return $query->where('type', $request->input('type'));
}, function ($query) {
    return $query->where('type', 1);
})
->get();