FIX: Checkboxes weren't always being set properly.

Note to all, `set()` values for computed properties must return the
new value the same as `get()` does.

app/assets/javascripts/admin/components/site-setting.js.es6

@@ -34,6 +34,7 @@ export default Ember.Component.extend(BufferedContent, ScrollTop, {
     },
     set(value) {
       this.set('buffered.value', value ? 'true' : 'false');
+      return value;
     }
   },