[BUG] EDI/X12 ISA segment does not round trip

[mgaffigan]

Repro:

1. Decode a valid X12 message with blank ISA-2 to xml
2. Encode the XML to EDI

Expected:
The resultant message is valid

Actual:
The ISA segment is the wrong length (spec requires 106 bytes).

Example:

ISA*00*          *00*          *ZZ*SUBMITTERID    *ZZ*PAYERID        *260330*1430*^*00501*000000001*0*T*:~