net/http: prevent transport sends two "Connection: close" headers

There are three functions that do Connection header write:
1. transport.go/ persistConn.roundTrip
2. transfer.go/ transferWriter.writeHeader
3. request.go/ Request.write

The root cause is roundTrip didn't lookup into request.Close and
transferWriter
didn't take care of extraHeaders.

Fixes #28886

Change-Id: I1d131019c7cd42eb1bcc972c631b7df7511c1f39
Reviewed-on: https://go-review.googlesource.com/c/150722


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