Fix invalid usage of import/export (adopted-ember-addons#221)

The combination of the way object-transforms.js is authored and the way it's consumed within this addon is not actually valid Javascript! Importing the default export from a module *does not* give you all the individual named imports in a handy container.

The code only happens to work because Ember's loader is being lenient and doesn't really follow the ECMA module spec.

But this code will break under any spec-compliant module loader, including embroider.

addon/utils/object-transforms.js

@@ -45,3 +45,7 @@ export function isPresent(objectInstance) {
 
   return !!keys.length;
 }
+
+export default {
+  compact, without, only, isPresent
+}