Fix DATETIMEOFFSET formatting to preserve fractional seconds.

Fix DATETIMEOFFSET formatting to preserve fractional seconds.

Fixed issue where a timezone-aware ``datetime`` value being converted to
string for use as a parameter value of a :class:`.mssql.DATETIMEOFFSET`
column was omitting the fractional seconds.

Fixes: #5045
Closes: #5046
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5046
Pull-request-sha: 99dc7b23f69b7b068795a02d20b88bf352c7dcd7

Change-Id: I971629466fe0675536bbdf55693f0c1821dfb3cc
(cherry picked from commit 0d316e42f0645283c222f050c403c9c8d20463ec)