// splitHeaderBlock splits headerBlock into fragments so that each fragment fits
// in a single frame, then calls fn for each fragment. firstFrag/lastFrag are true
// for the first/last fragment, respectively.
func http2splitHeaderBlock(ctx http2writeContext, headerBlock []byte, fn func(ctx http2writeContext, frag []byte, firstFrag, lastFrag bool) error) error {
	// For now we're lazy and just pick the minimum MAX_FRAME_SIZE