Fixed binary search algorithm bug and tests

The 'middle' variable inside the searchTimer function was computed incorrectly. This was breaking the phantom.js tests in ember (emberjs/ember.js#4564) when upgrading to the latest backburner.
This wasn't caught by the tests because the tests were also changed in the commit 2a5534a4f6f2340c1c727c87b357926a5688ce36.