Ember.String: Serialize objects with JSON.stringify in fmt()

If an argument of Ember.String.fmt() is an object, serialise it to
string with JSON.stringify() instead of .toString().

This commit also adds tests for the serialisation of `null` (to "(null"))
and `undefined` to an empty string.

I've made this change as a [recent commit on ember-data](https://github.com/emberjs/data/commit/b0be92515de73f5eca5a5412b3bdf690f1938b1a) only prints out [object Object], which is not that helpful.