Change the event for updating header

When using the apple inserter for emojis
there is no keyup event and the new value was not
set on model and thus not saved.

[MAILPOET-3564]

assets/js/src/newsletter_editor/components/heading.js

@@ -16,8 +16,8 @@ Module.HeadingView = Marionette.View.extend({
   },
   events: function () { // eslint-disable-line func-names
     return {
-      'keyup .mailpoet_input_title': _.partial(this.changeField, 'subject'),
-      'keyup .mailpoet_input_preheader': _.partial(this.changeField, 'preheader'),
+      'change .mailpoet_input_title': _.partial(this.changeField, 'subject'),
+      'change .mailpoet_input_preheader': _.partial(this.changeField, 'preheader'),
       'change #mailpoet_heading_email_type': (event) => {
         App.getChannel().trigger('changeWCEmailType', event.target.value);
       },