Tests failing in February (QueryDqlFunctionTest::testDateAdd with "month" dataset)

[Majkl578]

https://travis-ci.org/doctrine/doctrine2/jobs/336054843#L616

1) Doctrine\Tests\ORM\Functional\QueryDqlFunctionTest::testDateAdd with data set "month" ('month', 1, 2592000, 172800) Failed asserting that 2378089 matches expected 2592000.

The "month" dataset for QueryDqlFunctionTest::testDateAdd works with assumption that month has 30 days +- delta 2 days, but in February the difference is larger (2378089s is ~27.52 days).
https://github.com/doctrine/doctrine2/blame/master/tests/Doctrine/Tests/ORM/Functional/QueryDqlFunctionTest.php#L351

Test added in #6742.
Applies to 2.6 & master.

/cc @lcobucci