[5.5] Fix HasManyThrough relation with custom keys when used in whereHas

[maxhumme]

Hello,

When I use a HasManyThrough relation with a custom localKey in a Eloquent whereHas statement, it returns 0 results. This is because in the HasManyThrough class the primary key of the farParent model is used, instead of the in the relation defined custom key of that model.

To fix this, we go from

public function getExistenceCompareKey()
{
    return $this->farParent->getQualifiedKeyName();
}

to

public function getExistenceCompareKey()
{
    return $this->farParent->getTable().'.'.$this->localKey;
}

Test included in the commit.