Add a test to show what happens when a long http request is canceled

See https://github.com/libgdx/libgdx/pull/1287#issuecomment-47743225 for a description of the problem.

To see the problem, click the "GET Large" button, and then try canceling or clicking on another button to start another request before the first finishes.

The application will be blocked until the first download completes.