Fix regression in timesheet_period change.

addedit.php

@@ -165,6 +165,7 @@ if (intval($obj->timesheet_worked)) {
     // or in case of mouse click on the delete icon it will set to '1' by javaScript (delete object with given timesheet_id)
     echo '<input type="hidden" name="del" value="0" />';
     echo '<input type="hidden" name="stat" value="0" />';
+    echo '<input type="hidden" name="workperiod" value="0" />';
 
     // the value of timesheet_id will be the id of the timesheet to edit
     // or in case of addition of a new timesheet it will contain '0' as value