func (p *PUBREL) setFixedHeader() {
	p.fixedHeader = append(p.fixedHeader, TypePUBREL<<4|0x02)
	p.appendRemainingLength()
}
setFixedHeader sets the fixed header to the Packet.
package packet
const lenPUBRELFixedHeader = 2
const lenPUBRELVariableHeader = 2
type PUBREL struct {
	base
	PacketID uint16
}