Fix reference to product object property

[LukaszFormela]

There is a discrepancy between JSON returned from fetch('/products/PRODUCT_HANDLE.js') and {{ product | json }}.

In first one, options property contains array of objects:

options: [
   {name: "Option1", position: 1, values: Array(4)},
   {name: "Option2", position: 2, values: Array(4)},
   {name: "Option3", position: 3, values: Array(3)},
]

The latter returns options property that contains direct option names:

options: [
   'Option1',
   'Option2',
   'Option3',
]

To prevent errors being thrown while using this in conjunction with theme-product-form.js on JSON returned with liquid tag, we need to do a quick check on what type of options property does current product object contain.