Resolve "substitution planning "cancelled" checkbox does not have false as value when unchecked actively"

Closes #241 (closed)

aleksis/apps/chronos/frontend/components/AmendLesson.vue

@@ -154,11 +154,15 @@ export default {
   },
   methods: {
     transformCreateData(item) {
+      let { cancelled, ...createItem } = item;
       return {
-        ...item,
+        ...createItem,
         amends: this.selectedEvent.meta.id,
         datetimeStart: this.selectedEvent.startDateTime.toUTC().toISO(),
         datetimeEnd: this.selectedEvent.endDateTime.toUTC().toISO(),
+        // Normalize cancelled, v-checkbox returns null & does not
+        // honor false-value.
+        cancelled: cancelled ? true : false,
       };
     },
     transformPatchData(item) {