Fixed an error that causes a null value to be added as the first value in the history.states array

When executing setCurrentState(stateObj) for the first time, the value of oldState is pushed to the history.states array with a value of null. This change checks the oldState variable for a null value before pushing it onto the history.states array and if it is null, it instead pushes the state object passed to setCurrentState().
![Screen Shot 2013-02-25 at 11 45 35 PM](https://f.cloud.github.com/assets/933567/195319/8d7a23d8-7fe8-11e2-8af9-9e9fdfdfc5ed.JPG)