Signal action as finished when data was send, not when popup was closed.

In my app I save an image temporary for sharing/opening in other apps and delete it after the action.

It works with your current version of TTOpenInAppActivity in iOS 7. However, in iOS 8 the timings seem to have changed. Now the image gets transferred to the other app *after* the popup was already dismissed. But since I deleted the image as soon as the action was signaled finished with `activityDidFinish:` there was nothing to send to the other app.

I fixed this by using the `documentInteractionController:didEndSendingToApplication:` delegate method instead, which seems to be the better choice anyways.