Expectation comes first
Merge pull request #18231 from untidy-hair/improve_a_test

Expectation comes first
Allow precision option for MySQL datetimes.
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