IE Network Instance encoding fix
According to 3GPP TS 29.244, section 8.2.4, the network instance field shall be encoded as an OctetString and shall contain an identifier which uniquely identifies a particular Network instance (e.g. PDN instance) in the UP function. It may be encoded as a Domain Name or an Access Point Name (APN) as per clause 9.1 of 3GPP TS 23.003 [2]