Add OrderedHash#invert to preserve order in ruby 1.8

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

activesupport/lib/active_support/ordered_hash.rb

@@ -147,6 +147,10 @@ def replace(other)
         self
       end
 
+      def invert
+        OrderedHash[self.to_a.map!{|key_value_pair| key_value_pair.reverse}]
+      end
+
       def inspect
         "#<OrderedHash #{super}>"
       end

activesupport/test/ordered_hash_test.rb

@@ -228,4 +228,10 @@ def test_update_sets_keys
     @updated_ordered_hash.update(:name => "Bob")
     assert_equal [:name],  @updated_ordered_hash.keys
   end
+
+  def test_invert
+    @ordered_hash = ActiveSupport::OrderedHash[[["foo", "FOO"], ["bar", "BAR"]]]
+    @inverted_ordered_hash = @ordered_hash.invert
+    assert_equal [["FOO", "foo"], ["BAR", "bar"]], @inverted_ordered_hash.to_a
+  end
 end