Improper encoding of reserved characters

This was [originally](https://github.com/haskell/cabal/issues/1579) reported as a Cabal bug, but I believe it belongs here:

> The value of HTTP_PROXY has as format
> http://user:pass@host:80/
>
> In the password special characters can occur, like @ and !.
> According to wiki (http://en.wikipedia.org/wiki/Percent-escape)
> these characters must be percent-encoded, like %40 and %21.
>
> Cabal doesn't accept percent-encoded characters.
> And cabal errs on these characters (if password is a@b, Cabal doesn't look to the right site, but >probably to "b@host:80" (error message is not clear enough to be certain)

The issue does not name a file or test; start by tracing how HTTP_PROXY values are parsed and reproduce a password containing percent-encoded reserved characters such as %40 and %21. Done should mean these characters are accepted and the request uses the intended host, with regression coverage for the proxy URL case.

Written by the indexing model from the issue text.