change ^ and $ anchors in regexp to \A and \z respectively

http://guides.rubyonrails.org/security.html#regular-expressions

actionpack/lib/action_view/helpers/asset_tag_helper.rb

@@ -390,7 +390,7 @@ def image_tag(source, options={})
         end
 
         if size = options.delete(:size)
-          options[:width], options[:height] = size.split("x") if size =~ %r{^\d+x\d+$}
+          options[:width], options[:height] = size.split("x") if size =~ %r{\A\d+x\d+\z}
           options[:width] = options[:height] = size if size =~ %r{\A\d+\z}
         end