Eliminate unnecessary File.exist? in compute_public_path when correct file extension given

Currently compute_public_path in asset_tag_helper.rb performs an unnecessary File.exist? check when a source with the correct file extension has been passed in.

e.g. stylesheet_path('theme.css') results in a File.exist? check for theme.css.css

The attached patch eliminates this File.exist? call whenever the extension of the specified source matches the specified extension. There are no additional tests as there is already test coverage for source paths using non-standard extensions and source paths without extensions.