net/http: clarify ErrBodyNotAllowed error message

It's usually due to writing on HEAD requests.

R=golang-dev, rsc, r, r
CC=golang-dev
https://golang.org/cl/6206106

src/pkg/net/http/server.go

@@ -31,7 +31,7 @@ import (
 // Errors introduced by the HTTP server.
 var (
 	ErrWriteAfterFlush = errors.New("Conn.Write called after Flush")
-	ErrBodyNotAllowed  = errors.New("http: response status code does not allow body")
+	ErrBodyNotAllowed  = errors.New("http: request method or response status code does not allow body")
 	ErrHijacked        = errors.New("Conn has been hijacked")
 	ErrContentLength   = errors.New("Conn.Write wrote more than the declared Content-Length")
 )