URL-encode the content-disposition suggested filename for XML export data.

URL-encode the content-disposition suggested filename for XML export data.

This fixes the filename encoding for non-pure-ASCII site names on IE 6/7, and may fix issues with other ASCIIish chars.
Safari 2/Mac messes up the filename either way, but percent codes are probably still better here than garbage characters. :)
IE/Mac and Safari 3/Win don't appear to pay attention to the content-disposition and still show the XML inline.