XMLHttpRequest test fixes

- abort-event-loadend.htm: changed to call abort() in onloadstart rather than in onreadystatechange; otherwise the test ends up with throwing InvalidStateError in send().
- getresponseheader-error-state.htm: corrected the URL composition part; without "//", location.protocol ("http:") tends to be interpreted as <protocol> + <host> + <path> by user agent, which leads the result to 404 error instead of CORS failure.