Don't pluralize the CollectionSerializer#root for #json_key

One of three constituents is used to provide the
CollectionSerializer's #json_key:

1) the :root option - controlled by the caller
2) the #name of the first resource serializer - the root or
   underscored model name
3) the underscored #name of the resources object - generally
   equivalent to the underscored model name of #2

Of the three, only the latter 2 are out of the callers control, and
only the latter two are expected to be singular by default. Not
pluralizing the root gives the caller additional flexibility in
defining the desired root, whether conventionally plural,
unconventionally plural (e.g. objects_received:) or singular.

lib/active_model/serializer/collection_serializer.rb

@@ -23,8 +23,7 @@ module ActiveModel
       end
 
       def json_key
-        key = root || serializers.first.try(:json_key) || object.try(:name).try(:underscore)
-        key.try(:pluralize)
+        root || derived_root
       end
 
       def paginated?
@@ -36,6 +35,13 @@ module ActiveModel
       protected
 
       attr_reader :serializers
+
+      private
+
+      def derived_root
+        key = serializers.first.try(:json_key) || object.try(:name).try(:underscore)
+        key.try(:pluralize)
+      end
     end
   end
 end