Fix belongsToMany relation

[mjauvin]

Follows on from #536.

I also added the test that was failing from october/october to make sure this actually fixes the problem.

[Denoder]

I hadn't updated my library in a bit (since my pull request). The when i updated to this release i ended up getting a lot of ambiguous errors:

i had to remove

    /**
     * Create a new query builder for the pivot table.
     *
     * This is an extension of Laravel's `newPivotQuery` method that allows `belongsToMany` and `morphToMany` relations
     * to have conditions.
     *
     * @return \Illuminate\Database\Query\Builder
     */
    public function newPivotQuery()
    {
        $query = parent::newPivotQuery();

        // add relation's conditions and scopes to the query
        $this->addDefinedConstraintsToQuery($query);

        return $query->join($this->related->getTable(), $this->relatedPivotKey, '=', $this->relatedKey);
    }

    ```

[bennothommo]

@teranode are you using either of those fields for conditions or scope filters when defining the relation?

[Denoder]

@bennothommo None of them uses conditions or scopes.

[mjauvin]

@teranode Can you show the relations definitions for the models exhibiting this error?

Also, the migration table for the pivot table would be helpful.

[mjauvin]

@teranode maybe opening a new issue for this would be better... you can reference this PR for reference.