xdrjson: AssetCode12 allows invalid JSON value that doesn't round trip

[leighmcculloch]

What version are you using?

v22.1.0

What did you do?

$ echo '"ABC"' \
    | stellar xdr encode --type AssetCode12 \
    | stellar xdr decode --type AssetCode12
"ABC\\0\\0"

What did you expect to see?

Expected that the call to encode would fail, and that "ABC" would be considered an invalid AssetCode12, since that type always maps to at least 5 characters in JSON so as to clearly distinguish with AssetCode4.

The expectation is that only "ABC\\0\\0" would round trip for AssetCode12 for this value.

XDR-JSON is intended to be like XDR, that there is only a single representation for each XDR value.

What did you see instead?

The inconsistent round-tripping.