Detect bad proxies that suppress encoded slashes

[Jermolene]

Some webservers and proxies don't like URLs that contain encoded slashes (%2F) - see #720 for background. As discussed there, I believe that this behaviour is a hangover from an earlier era when web servers/proxies thought that part of their role was to protect inexperienced PHP programmers.

Rather than working around this problem, I'd like to update the client so that it can detect 404's that didn't come from the server code, and warn the user appropriately.

[oparoz]

Links you might want to present to the user:

• https://httpd.apache.org/docs/current/mod/core.html#allowencodedslashes
• https://www.phusionpassenger.com/documentation/Users%20guide%20Apache.html#_passengerallowencodedslashes_lt_on_off_gt

[Jermolene]

Thanks @oparoz