How ever you can forget about creating entries with XML payload which is not supported. And, for POST operation, you should not specify the key. POST is basically CREATE operation , where you can create a new entry identified by a key. The key is not existing currently in the database. So you can not refer the same. For example, to cretae a book, build the JSON with the entity strucutre, fill all the data and POST with out specifying the key value. Hope you are handling CSRF also.
Sreehari