Description:
------------
When adding one year (new DateInterval('P1Y')) to a leap day (new DateTime('2016-02-29')) results in incorrect date. It should return 2017-02-28 but returns 2017-03-01 instead.

For comparison, MariaDB 10 returns the correct date when doing SELECT DATE_ADD(DATE('2016-02-29'), INTERVAL 1 YEAR);

Test script:
---------------
var_dump( (new \DateTime('2016-02-29'))->add(new \DateInterval('P1Y')) );

Expected result:
----------------
object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2017-02-28 00:00:00.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(15) "Europe/Helsinki"
}


Actual result:
--------------
object(DateTime)#1 (3) {
  ["date"]=>
  string(26) "2017-03-01 00:00:00.000000"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(15) "Europe/Helsinki"
}