Fixes #12866

net/http.Client returns some errors wrapped in a *url.Error. To avoid
the requirement to unwrap these errors to determine if the cause was
temporary or a timeout, make *url.Error implement net.Error directly.

Change-Id: I1ba84ecc7ad5147a40f056ff1254e60290152408
Reviewed-on: https://go-review.googlesource.com/15672Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>