net/http: strip escaped password from error

Using password that returns from User.Password() won't work in this case
because password in Userinfo already unescaped. The solution is uses
User.String() to escape password back again and then stringify it to error.

Fixes #31808

Change-Id: I723aafd5a57a5b69f2dd7d3a21b82ebbd4174451
Reviewed-on: https://go-review.googlesource.com/c/go/+/175018


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>