Fix for issue #28 handling of "id" when inserting or updating documents.

Hi,

Thanks for your hard work on this project.
I am using it for the first time in production and find issue #28 quite annoying as it litters my schema.
I have fixed it (passes unit tests) in this pull request. Please have a look - I (and others, too) would be very happy to see this issue fixed in the mongodb-adapter.

Thanks!