net/http: use DialWithConn method of socks.Dialer

This change uses the DialWithConn method of socks.Dialer to ensure that
the bundled SOCKS client implementation is agnostic to the behavior and
capabilities of transport connections.

Also updates the bundled golang.org/x/net/internal/socks at git rev
7594486. (golang.org/cl/110135)

Updates #25104.

Change-Id: I87c2e99eeb857f182ea5d8ef569181d4f45f2e5d
Reviewed-on: https://go-review.googlesource.com/110136


Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>