Fixed finding assets for absolute paths

I use asset pipeline, but it happens i use absolute path for my assets. 
For example i have image located at app/public/some-image.png. I can use image_tag image_tag("/some-image.png"), but when i use wicked_pdf_image_tag("/some-image.png") in development mode(Rails.configuration.assets.compile == false) exception is raised: undefined method `pathname' for nil:NilClass.

This commit fix that (wicked_pdf_image_tag and other wicked_pdf_*_tag helpers).