// In order to avoid including external entities when parsing the appcast (a potential security vulnerability; see https://github.com/andymatuschak/Sparkle/issues/169), we ask NSXMLDocument to "tidy" the XML first. This happens to remove these external entities; it wouldn't be a future-proof approach, but it worked in these historical versions of OS X, and we have a more rigorous approach for 10.7+.