[Form] Fixed failing tests for DateTimeToStringTransformer.

Tests were only failing at the end of the month. PHP uses current day if it is not passed. Since February was used in the test cases, date was being moved to the next month (February has less days than other months).

src/Symfony/Component/Form/Tests/Extension/Core/DataTransformer/DateTimeToStringTransformerTest.php

@@ -23,7 +23,7 @@ public function dataProvider()
             array('Y-m-d H:i', '2010-02-03 16:05', '2010-02-03 16:05:00 UTC'),
             array('Y-m-d H', '2010-02-03 16', '2010-02-03 16:00:00 UTC'),
             array('Y-m-d', '2010-02-03', '2010-02-03 00:00:00 UTC'),
-            array('Y-m', '2010-02', '2010-02-01 00:00:00 UTC'),
+            array('Y-m', '2010-12', '2010-12-01 00:00:00 UTC'),
             array('Y', '2010', '2010-01-01 00:00:00 UTC'),
             array('d-m-Y', '03-02-2010', '2010-02-03 00:00:00 UTC'),
             array('H:i:s', '16:05:06', '1970-01-01 16:05:06 UTC'),