Bug fix for ArraySerializer json_key

When the resource is a zero result query,
i.e. post_comments = PostComment.where("1=0")
the json_key will become 'postcomments' rather than 'post_comments'.
Using 'underscore' instead of 'downcase' fixes the error.

Author: jiajiawang

lib/active_model/serializer/array_serializer.rb

@@ -29,7 +29,7 @@ def json_key
         if @objects.first
           @objects.first.json_key.pluralize
         else
-          @resource.name.downcase.pluralize if @resource.try(:name)
+          @resource.name.underscore.pluralize if @resource.try(:name)
         end
       end