(Re)fixes bug where URLs are naively double escaped. 

See https://bugs.launchpad.net/sparkle/+bug/681912 and https://github.com/andymatuschak/Sparkle/commit/00b67269d69f311ce4dcc39437f1d8ac1825b383 for the original fix. This seems to have been accidentally reverted in this merge: https://github.com/pornel/Sparkle/commit/9937e5a17b7149655268e8d7ebbede265b27a033 .