net/http: do not send redundant Connection: close header in HTTP/1.0 responses

net/http: do not send redundant Connection: close header in HTTP/1.0 responses

HTTP/1.0 connections are closed implicitly, unless otherwise specified.

Note that this change does not test or fix "request too large" responses.
Reasoning: (a) it complicates tests and fixes, (b) they should be rare,
and (c) this is just a minor wire optimization, and thus not really worth worrying
about in this context.

Fixes #5955.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12435043