Images aren't being constrained when the constrain filter is used

[nickendle]

When using the constrain filter on any image, like so:

<img src="{{ product.image | product_image_url | constrain: 560,560 }}">

The image URL that's returned doesn't include the parameters anymore:

http://images.bigcartel.com/product_images/100023343/0000_BIG_CARTEL_PRODDY_IMG_7614.jpg