wrong decode and encode for pdufield.UDHList

[skill215]

I found that the decode and encode (Bytes(), Len(), SerializeTo()) method for pdufield.UDHList were wrong.
According to 3GPP TS 23.040 Section 9.2.3.4. https://www.etsi.org/deliver/etsi_ts/123000_123099/123040/16.00.00_60/ts_123040v160000p.pdf Each IEI data inside the UDH should not have tailing zeros (0x00)
So we should change following definition

// UDH is a PDU field used for user data header.
type UDH struct {
	IEI      Fixed
	IELength Fixed
	IEData   Variable
}

To:

 type UDH struct {
	IEI      Fixed
	IELength Fixed
	IEData   Raw
}

And redefine the Raw type

[fiorix]

Happy to review a PR