net/http: close Body in client code always, even on errors, and document

Fixes #6981

LGTM=rsc
R=golang-codereviews, nightlyone
CC=adg, dsymonds, golang-codereviews, rsc
https://golang.org/cl/85560045

src/pkg/net/http/request.go

@@ -867,3 +867,9 @@ func (r *Request) wantsHttp10KeepAlive() bool {
 func (r *Request) wantsClose() bool {
 	return hasToken(r.Header.get("Connection"), "close")
 }
+
+func (r *Request) closeBody() {
+	if r.Body != nil {
+		r.Body.Close()
+	}
+}