Default the style to default style in #s3_protocol method of S3 storage

Thank you @eric-hu for reporting this regression.

Closes #583

lib/paperclip/storage/s3.rb

@@ -153,7 +153,7 @@ module Paperclip
         end
       end
 
-      def s3_protocol(style)
+      def s3_protocol(style = default_style)
         if @s3_protocol.is_a?(Proc)
           @s3_protocol.call(style)
         else