updated version
Merge branch 'master' of https://github.com/mjpete3/x12
updated one-off 835oa file
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added 276 / 277 messages
fixed bug in 835.xml
Marked some fields as optional

Certain fields are optional in certain instances on the 837P depending on version of the standard/position of the field. Made them optional so we don't end up sending blank fields.