[BUG][FIX] DateAndTime>>printOn: does not print decimal point for fractional seconds ( [cd][er][su][et][sm] Recommend including )

[tomkoenig at mindspring.com]

Recommend accepting this change.  It certainly makes things more
readable.
A one line change with one test case added.
Tested Squeak3.8b-6374. Test provided failed without, and passed with
fix.  All of the few hundred other date, time related sunit tests still
pass.(amazing)