Birthdays: Fix outgoing federation of birth dates

Signed-off-by: marcin mikołajczak <git@mkljczk.pl>

lib/pleroma/web/activity_pub/views/user_view.ex

@@ -93,8 +93,8 @@ def render("user.json", %{user: user}) do
       end
 
     birthday =
-      if user.show_birthday,
+      if user.show_birthday && user.birthday,
-        do: user.birthday,
+        do: Date.to_iso8601(user.birthday),
         else: nil
 
     %{