net/http: make Transport.RoundTrip return raw Conn.Read error on peek failure

From at least Go 1.4 to Go 1.6, Transport.RoundTrip would return the
error value from net.Conn.Read directly when the initial Read (1 byte
Peek) failed while reading the HTTP response, if a request was
outstanding. While never a documented or tested promise, Go 1.7 changed the
behavior (starting at https://golang.org/cl/23160).

This restores the old behavior and adds a test (but no documentation
promises yet) while keeping the fix for spammy logging reported in #15446.

This looks larger than it is: it just changes errServerClosedConn from
a variable to a type, where the type preserves the underlying
net.Conn.Read error, for unwrapping later in Transport.RoundTrip.

Fixes #16465

Change-Id: I6fa018991221e93c0cfe3e4129cb168fbd98bd27
Reviewed-on: https://go-review.googlesource.com/25153


Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>