Set Protocol when asset_path(source) starts with // 

Check to see if a calculated asset path begins with // if it does concatenate http: to the front or w/e default protocol the dev desires.  

I did a minor refactor that let me test it easier (not too familiar with minitest) but abstracted the check for rails asset compilation or an absolute path to a function.  

The purpose of this is to allow action_controller.asset_host to be a protocol relative url and wicked_pdf will just work.