Bug 1050396 - Properly handle zero duration in ring command.

As far as I could see, using a fake timer handles that for us -- acording to the Sinon docs, "Fake timers is a synchronous implementation of setTimeout and friends", so those calls inside `onsuccess` and `onerror` are made while inside `fakeClock.tick`, synchronously. The tests consistently pass locally and Gaia-Try is happy, so I think we're good.