Filter response headers, fix responseType's error in XHR

Passes a couple of wpt tests ([getallresponseheaders-cookies.htm](https://github.com/w3c/web-platform-tests/blob/master/XMLHttpRequest/getallresponseheaders-cookies.htm), [getresponseheader-cookies-and-more.htm](https://github.com/w3c/web-platform-tests/blob/master/XMLHttpRequest/getresponseheader-cookies-and-more.htm), [responsetype.htm](https://github.com/w3c/web-platform-tests/blob/master/XMLHttpRequest/responsetype.html))
Blocks #2525