* fix typo

* factorize update

* remove useles if() {}

* fix injection test with array of only one item

* factorize add

* if multiple operation sent, the response should for multiple items, no
matter only one item is added, updated or deleted

* fix returns

* sanitize input

* fix responses bodies; update tests

* update tests

* update documentation

* fix typo

* update XMLRPC API

* DELETE must return a collection, no matter a single item is requested

* update unit tests