fix birthday informations from facebook: birthday is empty if permiss…

[marvasDE]

…ion is denied (therefore it was be converted to current date) + fix date format

Facebook's date format is american (MM/dd/yyyy) and have to be given to the date method to convert correctly. See the comment from chougron in the old PR #22, which had the same issue.

[sprankhub]

I am not sure if the date needs to be converted to a different format. However, checking for an empty birthday definitely makes sense and fixes the issue that the birthday is set to the current date.

[Marko-M]

Thanks, fixed with #81

[sprankhub]

#81 actually just fixed the "date of birth set to current date issue". I am not sure if the date conversion proposed in this PR needs to be included. Our app is not allowed to get the date of birth, so I cannot test. However, if the API returns the format as defined in https://developers.facebook.com/docs/graph-api/reference/user/ under birthday, the format is indeed either MM/DD/YYYY or MM/DD or YYYY. So I guess some conversion needs to be done.