Fixed #14453 -- Changed handling of microseconds part in typecast_timestamp()...

Fixed #14453 -- Changed handling of microseconds part in typecast_timestamp() DB backend helper function to be more correct. Thanks philipn for the report and fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14664 bcc190cf-cafb-0310-a4f2-bffc1f526a37