Fixed calling ``Item.partial_save`` on a new record now correctly saves.

Per the docs (http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_UpdateItem.html), calling ``Item.partial_save()`` in DDBv2 should either alter an existing item or create a new one. The former works fine, but the latter would cause an error, as reported in #1521. This fixes that.

Review would be appreciated. @garnaat @jamesls @danielgtaylor