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

@@ -51,6 +51,8 @@ module Middleman
               end
             end
           end
+          
+          params = params.delete_if {|key| key == :supported_extensions }
 
           super(path, params)
         end