net/textproto: add "Cookie" to the common headers list

Seems to have been omitted before.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/7749049

src/pkg/net/textproto/reader.go

@@ -574,6 +574,7 @@ var commonHeaders = []string{
 	"Content-Length",
 	"Content-Transfer-Encoding",
 	"Content-Type",
+	"Cookie",
 	"Date",
 	"Dkim-Signature",
 	"Etag",