Support for optional data (4.19) of the XDR spec

[fdawg4l]

I tried decoding optional-data 4.19 and came across this comment in the code

https://github.com/davecgh/go-xdr/blob/master/xdr2/decode.go#L572

// RFC Sections 4.15 though 4.19 only apply to the data specification language
// which is not implemented by this package

I don't understand the "only apply to the data specification language" bit. The API I'm using (NFSv3, to be specific) is encoding a linked-list with the above optional-data format and other than decoding each element and then checking for the "follows" 4B for zero before terminating (which is what I'm currently doing here), is there a way to decode this data?

If the loop-and-check method is the only way, are you averse to me pushing a PR to add support for this? I'm thinking it would require a StructTag to descend down the right path for a slice. Let me know and I'll be happy to get started on that.

Thanks!

[fdawg4l]

I see now there's already a PR open for this. Closing.