Allow Eloquent Builder wheres with closures

Currently if you do `MyModel::where(function($q) { ... });` $q will be an instance of Illuminate\Database\Query\Builder. This is unfortunate if you want to utilize Eloquent\Builder's functionality in the nested where. This fixes that.

Submitted to 4.1 instead of master as only one unit test had to be adjusted for obvious reasons, and I can't think of a case where swapping a query builder with an eloquent builder will break things.