support MongoDB extended JSON on ObjectID#to_json

lib/mongo/types/objectid.rb

@@ -142,8 +142,12 @@ module Mongo
     end
     alias_method :inspect, :to_s
 
-    def to_json(*args)
-      %Q("#{to_s}")
+    # Convert to MongoDB extended JSON format. Since JSON includes type information,
+    # but lacks an ObjectID type, this JSON format encodes the type using an $id key.
+    #
+    # @return [String] the object id represented as MongoDB extended JSON.
+    def to_json(escaped=false)
+      "{\"$oid\": \"#{to_s}\"}"
     end
 
     # @deprecated

test/test_objectid.rb

@@ -151,6 +151,6 @@ class ObjectIDTest < Test::Unit::TestCase
 
   def test_json
     id = ObjectID.new
-    assert_equal %Q("#{id}"), id.to_json
+    assert_equal "{\"$oid\": \"#{id}\"}", id.to_json
   end
 end