Fix storage of time.Time objects with non-local timezones in DATETIME columns.

Storing time.Time objects with non-local timezones in DATETIME columns didn't work with the mymysql driver: when the value was stored, timezone information was simply ignored; when the value was retrieved, it was assumed to be local time.  This commit fixes the problem, by converting all time.Time objects to the local timezone before sending them to the MySQL server.

I've included a test case to illustrate the problem and to verify that the problem is fixed by my change.