Revert "Change ObjectID to ObjectId"

This reverts commit efcc112397a8080fbfabd15dab30036bbb6c7140.

lib/mongo_mapper/bson_id.rb

 class String
   def to_id
-    BSON::ObjectId self
+    BSON::ObjectID self
   end
 end
-class BSON::ObjectId
+class BSON::ObjectID
   def to_id
     self
   end