add `XMLHttpRequest.getResponseHeader` to test fixtures.
add `XMLHttpRequest.getResponseHeader` to test fixtures.

tests now pass again.

erhanaydin/retinajs,gnepal7/retinajs,colorwalf/retinajs,sovanna/retinajs,Marcobaldessari/retinajs,sovanna/retinajs,erhanaydin/retinajs,ganeshaproduccions/retinajs,jeremymouton/retinajs,colorwalf/retinajs,sedx/retinajs,gnepal7/retinajs,Lori1072/retinajs,imulus/retinajs,casaaga/retinajs,Marcobaldessari/retinajs,ganeshaproduccions/retinajs,casaaga/retinajs,cy6erskunk/retinajs,imulus/retinajs,sedx/retinajs,jeremymouton/retinajs