cloneHeader duplicates what Header.clone() method is doing. It's
used in a single place, which can be replaced with the use of the
method.

Change-Id: I6e8bbd6c95063f31ca3695f13fa7478873230525
GitHub-Last-Rev: eb08aeae9b4257df5cf08d6d30903ddeec6b6242
GitHub-Pull-Request: golang/go#27817
Reviewed-on: https://go-review.googlesource.com/136762Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>