Fix broken tests and a bug about relations

##### `testWithBelongsTo()`

Should be broken only with some MongoDB config or context. By using `orderBy` it will work everywhere.

##### `testBelongsToMany()`

Some changes have been made in the Laravel Core Model class recently like this one : https://github.com/laravel/framework/commit/cd4e0ca54d42f7737e166b71dbbf7d8881323a5f. I don't completely understand what happened here but everything should be ok now !