Fixed document updating

If document ID in database is stored as ObjectIdHex it should be also in query (especially while updating so it won't be necessary to use [this](https://github.com/crackcomm/mora/blob/f71bcb2df43e29cd8db8d1badf481d48d962bd17/document_resource.go#L168)).
While updating document, stored ObjectIdHex `_id` field won't match string `_id` field in the query and new document will be created.