encode command ignores additional fields in xdr instead of failing

[sisuresh]

What version are you using?

stellar-xdr 20.1.0 (8b9d623ef40423a8462442b86997155f2c04d3a1)
xdr (+curr): b96148cd4acc372cc9af17b909ffe4b12c43ecb6
xdr (+next): 6a71b137bc49f901bed53c8c215c26213017026c

What did you do?

Ran stellar-xdr encode --type ConfigUpgradeSet xdr.json where xdr.json contained -

{
  "updated_entry": [
    {
      "contract_execution_lanes": {
        "ledger_max_tx_count": 100,
        "additional_setting_bad": 100
      }
    }
  ]
}

What did you expect to see?

Error message due to bad setting.

What did you see instead?

AAAAAQAAAAsAAABk (Which is the contract_execution_lanes xdr without additional_setting_bad).

This caused us some issues during the testnet reset because we added a new field to one of the settings and updated our JSON file, but my local stellar-xdr was out of date, so the encode command was just ignoring the new field instead of failing.