move OrderedSet to separate file

lib/active_model/ordered_set.rb

@@ -0,0 +1,25 @@
+module ActiveModel
+  class OrderedSet
+    def initialize(array)
+      @array = array
+      @hash = {}
+
+      array.each do |item|
+        @hash[item] = true
+      end
+    end
+
+    def merge!(other)
+      other.each do |item|
+        next if @hash.key?(item)
+
+        @hash[item] = true
+        @array.push item
+      end
+    end
+
+    def to_a
+      @array
+    end
+  end
+end

lib/active_model/serializer.rb

@@ -3,30 +3,6 @@ require "active_support/core_ext/module/anonymous"
 require "set"
 
 module ActiveModel
-  class OrderedSet
-    def initialize(array)
-      @array = array
-      @hash = {}
-
-      array.each do |item|
-        @hash[item] = true
-      end
-    end
-
-    def merge!(other)
-      other.each do |item|
-        next if @hash.key?(item)
-
-        @hash[item] = true
-        @array.push item
-      end
-    end
-
-    def to_a
-      @array
-    end
-  end
-
   # Active Model Serializer
   #
   # Provides a basic serializer implementation that allows you to easily

lib/active_model_serializers.rb

@@ -2,6 +2,7 @@ require "active_support"
 require "active_support/core_ext/string/inflections"
 require "active_support/notifications"
 require "active_model"
+require "active_model/ordered_set"
 require "active_model/array_serializer"
 require "active_model/serializer"
 require "set"