after a long sequence of little tests I have found out exactly the problem!
With the locale option setted on "it", Logstash works in case sensitive mode and don't recognize "Apr", instead "apr", yes!
If the locale is setted on "en", Logstash is not case sensitive! and works in every case!
It's seems to be a bug, because with the previous version of Logstash, the 1.4.2, i did not have problem (with the same data source), but now i'm working with Logstash 1.5.2.
One more thing, in the official documentation, the example works with the month's name letter in upper-case of timestamp data source.