http: return 413 instead of 400 when the request body is too large

RFC2616 says servers should return this status code when
rejecting requests that are too large.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.14

R=bradfitz
CC=golang-dev
https://golang.org/cl/4962041

src/pkg/http/serve_test.go

@@ -891,8 +891,8 @@ func TestRequestLimit(t *testing.T) {
 		// we do support it (at least currently), so we expect a response below.
 		t.Fatalf("Do: %v", err)
 	}
-	if res.StatusCode != 400 {
-		t.Fatalf("expected 400 response status; got: %d %s", res.StatusCode, res.Status)
+	if res.StatusCode != 413 {
+		t.Fatalf("expected 413 response status; got: %d %s", res.StatusCode, res.Status)
 	}
 }
 

src/pkg/http/server.go

@@ -572,7 +572,7 @@ func (c *conn) serve() {
 				// responding to them and hanging up
 				// while they're still writing their
 				// request.  Undefined behavior.
-				msg = "400 Request Too Large"
+				msg = "413 Request Entity Too Large"
 			} else if neterr, ok := err.(net.Error); ok && neterr.Timeout() {
 				break // Don't reply
 			}