Uses closed connection from a proxy (e.g. squid2)

This is related to #14, which may have not been fixed properly. When using `cabal update` built with `HTTP-4000.2.17` and using a squid2 proxy, I get `cabal: does not exist`. A friend of mine did some debugging and found out:

> it seems that cabal does a GET http://hackage.haskell.org/packages/archive/00-index.tar.gz
> squid responds with a cache hit 301 and closes the connection.
> cabal tries to use pipelining and fails
> it doesn't try a new connection

It does not happen with squid3, or perl’s `HTTP::Proxy`.

Comienza revisando el issue relacionado #14 y el flujo de la solicitud de cabal update descrito en este informe; después, reproduce el fallo con HTTP-4000.2.17 a través de un proxy squid2. Se considera completado cuando la solicitud ya no falla si squid cierra la conexión después de una respuesta 301 almacenada en caché.

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