test: use a distinct source string that resolves to an equal PlainDate

Addresses review feedback on #650: the equal-PlainDate test now sets a
different string that still resolves to the same PlainDate, so it exercises
value-equality rather than raw-string identity.

Author: momomuchu

tests/computed.test.ts

@@ -302,8 +302,8 @@ describe('Computed', () => {
 
         expect(handler).toHaveBeenCalledTimes(1);
 
-        // Setting to a string that resolves to an equal PlainDate should not re-notify.
-        source$.set('2024-01-15');
+        // Setting to a different string that resolves to an equal PlainDate should not re-notify.
+        source$.set('2024-01-15T10:30:00');
 
         expect(handler).toHaveBeenCalledTimes(1);
     });