http: change ResponseWriter.SetHeader(k,v) to Header() accessor

http: change ResponseWriter.SetHeader(k,v) to Header() accessor

Caller code needs to change:

rw.SetHeader("Content-Type", "text/plain")
to:
rw.Header().Set("Content-Type", "text/plain")

This now permits returning multiple headers
with the same name using Add:

rw.Header().Add("Set-Cookie", "..")
rw.Header().Add("Set-Cookie", "..")

This patch also fixes serialization of headers, removing newline characters.

Fixes #488
Fixes #914

R=rsc
CC=gburd, golang-dev
https://golang.org/cl/4239076