net/http: add additional status codes defined in RFC 6585

428 Precondition Required
429 Too Many Requests
431 Request Header Fields Too Large
511 Network Authentication Required

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7678043

src/pkg/net/http/status.go

@@ -51,6 +51,13 @@ const (
 	StatusServiceUnavailable      = 503
 	StatusGatewayTimeout          = 504
 	StatusHTTPVersionNotSupported = 505
+
+	// New HTTP status codes from RFC 6585. Not exported yet in Go 1.1.
+	// See discussion at https://codereview.appspot.com/7678043/
+	statusPreconditionRequired          = 428
+	statusTooManyRequests               = 429
+	statusRequestHeaderFieldsTooLarge   = 431
+	statusNetworkAuthenticationRequired = 511
 )
 
 var statusText = map[int]string{
@@ -99,6 +106,11 @@ var statusText = map[int]string{
 	StatusServiceUnavailable:      "Service Unavailable",
 	StatusGatewayTimeout:          "Gateway Timeout",
 	StatusHTTPVersionNotSupported: "HTTP Version Not Supported",
+
+	statusPreconditionRequired:          "Precondition Required",
+	statusTooManyRequests:               "Too Many Requests",
+	statusRequestHeaderFieldsTooLarge:   "Request Header Fields Too Large",
+	statusNetworkAuthenticationRequired: "Network Authentication Required",
 }
 
 // StatusText returns a text for the HTTP status code. It returns the empty