allows you to use image_tag helper and have it build the asset urls for
your srcset images, just like it does for your main src. Leaves absolute
urls alone, (having // in the path)
```
<%= image_tage 'pic_1980.jpg', srcset: 'pic_640.jpg 2x, pic_1024.jpg 3x' %>
=> <img src="/images/pic_1980.jpg" srcset="/images/pic_640.jpg 2x, /images/pic_1024.jpg 3x">
```
