Fixed document Update. (second argument is not just document)

When we are pushing `_id` (which we don't have to do) we are automatically make it impossible to use "atomic" MongoDB [Update Methods](http://docs.mongodb.org/manual/reference/operator/nav-update/#id1).

Error which occurs when trying to establish some changes in document using `$set` field:

> Invalid modifier specified: _id
Merge pull request #16 from crackcomm/patch-1

Fixed atomic document Update.   @crackcomm, can you provide an example of such a case?  want to add it to the functional tests (jmeter)