net/http: Don't write 'Connection: close' header multiple times.

net/http: Don't write 'Connection: close' header multiple times.

When writing the 'Connection: close' header based on response Close
attribute we also check if it is already in the headers scheduled
to be written and skip if necessary.

Fixes #19499

Change-Id: I92357344a37ae385454ec8006114fa4cfa585810
Reviewed-on: https://go-review.googlesource.com/38076
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>