MySQL 5.6 and later supports microsecond precision in datetime.

MySQL 5.6 supports microsecond fraction with datetime functions: http://dev.mysql.com/doc/refman/5.6/en/fractional-seconds.html

This patch allows saving microseconds to mysql's datetime columns such as`updated_at`. This is significant to take benefits from [cache_key second precision updated to nanoseconds](https://github.com/rails/rails/commit/900dbc54f17500e14e481eab2ebf88acfc5448aa).

You might want to branch it to include this only for 5.6, but passing these values to < 5.6 doesn't cause issues either, based on my experiment.

See also [a pull request on mysql2 gem](https://github.com/brianmario/mysql2/issues/324) that addresses retrieving microseconds *from* mysql to inflate to Time with typecasting enabled.