Use Vumi codecs for outbound SMPP messages.

[hodgestar]

We have special support for GSM 03.38 and UCS2 codecs for inbound SMPP messages, but we don't provide these for outbound SMPP messages currently.

[hodgestar]

The inbound SMPP message processor (DeliverShortMessageProcessor) sets up a Vumi codec class at https://github.com/praekelt/vumi/blob/develop/vumi/transports/smpp/processors/default.py#L253 and uses it at https://github.com/praekelt/vumi/blob/develop/vumi/transports/smpp/processors/default.py#L288.

The outbound SMPP message processor (SubmitShortMessageProcessor) should do the equivalent. We will need to replace https://github.com/praekelt/vumi/blob/develop/vumi/transports/smpp/processors/default.py#L510.

We will also need a test that GSM 03.38 works.