QUnit should keep a local reference to Date

[Krinkle]

From qunitjs/node-qunit@4252119:

Since modules like node-timekeeper can modify it, node-qunit protects against this. Right now it does that by hacking the following line into qunit.js. Perhaps we should put it in the main repo here?

    Date = window.Date;

[jzaefferer]

First time I hear of that issue, but I don't see a problem with patching that. PR?