[5.3] fix withCount aliasing problem

[moritz-h]

Using withCount with multiple tables and the aliasing feature only for the first entry leads to wrong result names.
$model->withCount(['foo as foo_bar', 'foo']);

Error is here:
https://github.com/laravel/framework/blob/5.3/src/Illuminate/Database/Eloquent/Builder.php#L1112
The code is in an foreach loop so isset($alias) is also true when the last loop set the alias but the current loop does not us an alias. So the name from the last loop is reused instead of the default name.