go.net/ipv4: add Flags field to Header

Fixes golang/go#6453.

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/13832043
diff --git a/ipv4/header_test.go b/ipv4/header_test.go
index cc2fd8e..121d1f2 100644
--- a/ipv4/header_test.go
+++ b/ipv4/header_test.go
@@ -16,28 +16,28 @@
 var (
 	wireHeaderFromKernel = [ipv4.HeaderLen]byte{
 		0x45, 0x01, 0xbe, 0xef,
-		0xca, 0xfe, 0x05, 0xdc,
+		0xca, 0xfe, 0x45, 0xdc,
 		0xff, 0x01, 0xde, 0xad,
 		172, 16, 254, 254,
 		192, 168, 0, 1,
 	}
 	wireHeaderToKernel = [ipv4.HeaderLen]byte{
 		0x45, 0x01, 0xbe, 0xef,
-		0xca, 0xfe, 0x05, 0xdc,
+		0xca, 0xfe, 0x45, 0xdc,
 		0xff, 0x01, 0xde, 0xad,
 		172, 16, 254, 254,
 		192, 168, 0, 1,
 	}
 	wireHeaderFromTradBSDKernel = [ipv4.HeaderLen]byte{
 		0x45, 0x01, 0xdb, 0xbe,
-		0xca, 0xfe, 0xdc, 0x05,
+		0xca, 0xfe, 0xdc, 0x45,
 		0xff, 0x01, 0xde, 0xad,
 		172, 16, 254, 254,
 		192, 168, 0, 1,
 	}
 	wireHeaderToTradBSDKernel = [ipv4.HeaderLen]byte{
 		0x45, 0x01, 0xef, 0xbe,
-		0xca, 0xfe, 0xdc, 0x05,
+		0xca, 0xfe, 0xdc, 0x45,
 		0xff, 0x01, 0xde, 0xad,
 		172, 16, 254, 254,
 		192, 168, 0, 1,
@@ -51,6 +51,7 @@
 		TOS:      1,
 		TotalLen: 0xbeef,
 		ID:       0xcafe,
+		Flags:    ipv4.DontFragment,
 		FragOff:  1500,
 		TTL:      255,
 		Protocol: 1,