Setting XHR response type fails due to xhr.responseType being modified before XHR is opened

http-browserify accepts a 'responseType' parameter to set the XHR response type. However, in Firefox 31 at least, the logic to set the response type always fails because it is set before xhr.open() is called.

The call to set 'xhr.responseType = params.responseType' fails with 'An attempt was made to use an object that is not, or is no longer, usable' ala. http://stackoverflow.com/questions/13216903/get-binary-data-with-xmlhttprequest-in-a-firefox-extension