Corrected image_tag helper so that supported_extensions doesn't appear in the tag attributes

The modifications to add FastImage support made it that supported_extensions="[.png .jpg .jpeg .bmp .gif]" appeared in the img tag attributes upon building the project.

middleman-more/lib/middleman-more/extensions/automatic_image_sizes.rb

@@ -52,6 +52,8 @@ module Middleman
             end
           end
           
+          params = params.delete_if {|key| key == :supported_extensions }
+
           super(path, params)
         end
       end