67 check mount when setting state

Fixes #67
The only reason this test fails on the old code is because React uses the `isMounted` internally when throwing that invariant error. If that changes in the future this test will no longer be relevant. This test is now coupled to the implementation of React.

Use `React.unmountComponentAtNode` to actually unmount the component and check if everything is ok.

Doing the above, you'll find out something very interesting. `this.isMounted()` returns `true` in `componentWillUnmount` http://jsbin.com/yemir/1/edit?js,console,output. Calling `setState` here will throw an invariant error. Therefore, you cannot guard that `setState` call with `isMounted()`. The new code would still crash in production. Since this test wasn't written correctly, you could not have caught this scenario.

Now back to the drawing board. Maybe we can figure out if the request was aborted by checking the error code inside the callback and only do `setState` if the request wasn't aborted?