Clear spoiler when replying to toot that doesn't also have a spoiler (#1662)

app/assets/javascripts/components/reducers/compose.jsx

@@ -157,6 +157,9 @@ export default function compose(state = initialState, action) {
       if (action.status.get('spoiler_text').length > 0) {
         map.set('spoiler', true);
         map.set('spoiler_text', action.status.get('spoiler_text'));
+      } else {
+        map.set('spoiler', false);
+        map.set('spoiler_text', '');
       }
     });
   case COMPOSE_REPLY_CANCEL: