Format the datetime string according to the precision of the datetime field.

Incompatible to rounding behavior between MySQL 5.6 and earlier.

In 5.5, when you insert `2014-08-17 12:30:00.999999` the fractional part is ignored. In 5.6, it's rounded to `2014-08-17 12:30:01`: http://bugs.mysql.com/bug.php?id=68760