VKontakte birthdate handling fix

According to the documentation for developers at vk.com/developers.php
http://vk.com/developers.php?oid=-1&p=%D0%9E%D0%BF%D0%B8%D1%81%D0%B0%D0%BD%D0%B8%D0%B5_%D0%BF%D0%BE%D0%BB%D0%B5%D0%B9_%D0%BF%D0%B0%D1%80%D0%B0%D0%BC%D0%B5%D1%82%D1%80%D0%B0_fields

bdate
Date is shown in the following format: "23.11.1981" or "21.9" (if the year is hidden). If the full birthdate is hidden, then when receiving data in XML format, the tag "bdate" will be missing from the node <user>.