Fix bug product with variations

Fix bug with product and variations, see screen: http://d.pr/i/14NL2/3uBBcGaF

I don't know if there is some specific reason, but the wc_get_product_terms function return list with only names and not the object (this if there isn't any "fields" specified in arguments of function), as expected by variable.php template at line 45. I think it's a mistake.