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)

El issue no nombra ningún archivo ni test; empieza rastreando cómo se analizan los valores de HTTP_PROXY y reproduce una contraseña que contenga caracteres reservados codificados porcentualmente, como %40 y %21. Debe considerarse terminado cuando estos caracteres se acepten y la solicitud use el host previsto, con cobertura de regresión para el caso de la URL del proxy.

Escrito por el modelo de indexación a partir del texto del issue.