Fix the round-trip handling. This is still missing for message
types in other directions as this would require me writing test
cases first. I will get to that.
Done on the flight to iceland, comitted in the bus to the hotel. It
is a bit rushed as the battery runs low. Updated the test result as
the RP-UserData was short in the other data
It is not clear of how much parsing/handling code can be shared with
the GSM04.08 code. There are also a lot less IEs to handle for SMS so
we might not need the fixed size/data holder classes.