Fix `xhr.restore()` in JS test

This test was failing sporadically -- in my testing, I discovered that when it failed, it did so because `this.xhr` was the [`FakeXMLHttpRequest`](http://sinonjs.org/docs/#FakeXMLHttpRequest) class, and had no `restore` method to call on it. However, `this.xhrRestore` did exist, and I hypothesize that something about the test setup and cleanup was behaving weirdly because one test saved just this method, rather than the full `xhr` object. This pull request seems to make the problem go away, although I still don't understand *why* the test was failing. If this test turns out to be still failing sporadically again, it should be disabled again.

Reviewers: @jzoldak or @wedaly.