Add a test to catch that nobody rearranges fields in FrameHeader

So its size doesn't bloat because of alignment requirement.

frame_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"reflect"
 	"testing"
+	"unsafe"
 )
 
 func testFramer() (*Framer, *bytes.Buffer) {
@@ -15,6 +16,13 @@ func testFramer() (*Framer, *bytes.Buffer) {
 	return NewFramer(buf, buf), buf
 }
 
+func TestFrameSizes(t *testing.T) {
+	// Catch people rearranging the FrameHeader fields.
+	if got, want := int(unsafe.Sizeof(FrameHeader{})), 12; got != want {
+		t.Errorf("FrameHeader size = %d; want %d", got, want)
+	}
+}
+
 func TestWriteRST(t *testing.T) {
 	fr, buf := testFramer()
 	var streamID uint32 = 1<<24 + 2<<16 + 3<<8 + 4