Fix [Bug 1190463] [NEW] Bulk Edit - Clear Custom Date Columns.

Problem arose because Qt doesn't emit a changed signal if you set the
widgets value to what it currently contains.

src/calibre/gui2/custom_column_widgets.py

@@ -156,6 +156,7 @@ class DateTimeEdit(QDateTimeEdit):
         self.setDateTime(now())
 
     def set_to_clear(self):
+        self.setDateTime(now())
         self.setDateTime(UNDEFINED_QDATETIME)
 
     def keyPressEvent(self, ev):