Added to_json for Mongo::ObjectID.

lib/mongo/types/objectid.rb

@@ -100,6 +100,10 @@ module Mongo
     end
 
     def inspect; to_s; end
+    
+    def to_json(*args)
+      %Q("#{to_s}")
+    end
 
     # Get a string representation of this ObjectID using the legacy byte
     # ordering. This method may eventually be removed. If you are not sure

test/test_objectid.rb

@@ -1,4 +1,4 @@
-require 'test/test_helper'
+require 'test_helper'
 
 class ObjectIDTest < Test::Unit::TestCase
 
@@ -139,4 +139,9 @@ class ObjectIDTest < Test::Unit::TestCase
 
     assert_in_delta time.to_i, id.generation_time.to_i, 2
   end
+  
+  def test_json
+    id = ObjectID.new
+    assert_equal %Q("#{id}"), id.to_json
+  end
 end