SBE 2.0 features in 1.0 spec

[mjpt777]

It seems the message header has the fields for group count and variable length field in the message header from 2.0 in the 1.0 spec.

https://github.com/FIXTradingCommunity/fix-simple-binary-encoding/blob/master/v1-0-STANDARD/doc/03MessageStructure.md

[kleihan]

@mjpt777 thank you for catching that. It seems that the error occurred on February 28, 2017 when the branch "v2-0-issue26" was merged into the v1.0 DRAFT files. It looks like we will have to issue another errata version of SBE v1.0 after having reverted those changes.