add product variant to cart without attributes/values

Now I've finally played with attributes and attribute_values and it hurt. The join in this patch results in one attribute name per attribute but multiple attribute values.

It works and gives us something to think about.
so
```
$name = $result->attribute->name;
```
which is nice and clean - just one result but for the value we get a resultset hence the 'first' in this:
```
 but my $value = $result->product_attribute_values->first->attribute_value->value;
```
this sucks