net/http: make Transport send 'Connection: close' when DisableKeepAlives

No bug was open, but I found an old email to myself to investigate
when I suspected this was happening.

Change-Id: Icedefec6f15a000eaabb2693b0640b3b6c8bf82c
Reviewed-on: https://go-review.googlesource.com/1578


Reviewed-by: Andrew Gerrand <adg@golang.org>