message: Fix for onblock abi serialization
diff: "name": "block_header",
  "base": "",
  "fields": [
  - {"name":"previous", "type":"uint8[]"},
  + {"name":"previous", "type":"checksum256"},
  {"name":"timestamp", "type":"uint32"},
  - {"name":"transaction_mroot", "type":"uint8[]"},
  - {"name":"action_mroot", "type":"uint8[]"},
  - {"name":"block_mroot", "type":"uint8[]"},
  + {"name":"transaction_mroot", "type":"checksum256"},
  + {"name":"action_mroot", "type":"checksum256"},
  + {"name":"block_mroot", "type":"checksum256"},
  {"name":"producer", "type":"account_name"},
  {"name":"schedule_version", "type":"uint32"}
  ]
  