Use alternative download method for non RFC2231-capable browsers

wget, IE, google chrome users will love us if we implement the following workaround for the browsers that do not support RFC2231:

updown receives the request for a file, say a public one '/p/abc' that goes to /public/abc
updown looks up the file name and constructs Content-Disposition: attachment header with the real 'readme.txt' filename
However, since the browser is not in RFC2231 whitelist this header will be ignored and the end user will get 'abc' file downloaded with readme.txt content.
So we will simply url-escape utf-8 representation of the filename and create a redirect request that goes to /public/abc/readme.txt just appending the filename to the unique identifier. All browsers will be much happier then.

For this to happen we need to fix LP:556306 which looks like apache mod_rewrite rule issue and then fix updown to produce such kind of URL redirects.