http: change most map[string][]string types to new Values type

This replaces most the map[string][]string usage with
a new Values type name, with the usual methods.

It also changes client.PostForm to take a Values, rather
than a map[string]string, closing a TODO in the code.

R=rsc
CC=golang-dev
https://golang.org/cl/4532123
7 files changed