prevent image_tag from overriding absolute paths

Using image_tag for non-asset files, e.g. carrierwave uploads, wouldn't work because half-pipe adjusts the path to always be /assets/images/image.png.

This is a very small adjustment to prevent this if the path begins with a forward slash, which is part of Rails' default implementation. After browsing through the source for Rails image_tag, I feel that there might be some value to deferring more readily to Rails here as it handles a lot of different cases such as full URLs that I worry half-pipe might override.