Adding Change Node tests

Tests to cover the Change Node. I didn't feel an urge to factor out the common helper.load() function this time as it's not quite as repetitive as in the case of the Switch Node. Please let me know if I shall add utility methods. I personally don't think it's necessary here.
Would be more succinct to say:

    msg.should.not.have.property('payload');

And get rid of the try/catch