[webauthn] challenge in ClientData should be ArrayBuffer or BufferSource ?

equalsJeffH has just created a new issue for 
https://github.com/w3c/webauthn:

== challenge in ClientData should be ArrayBuffer or BufferSource ? ==
https://w3c.github.io/webauthn/#sec-client-data
```dictionary ClientData {
    required DOMString           challenge;    <<==  should be 
ArrayBuffer or BufferSource ?
    [...]
};
```
I *think* ArrayBuffer is more appropriate in ClientData than 
BufferSource, but I'm not sure, and I'm not sure it actually matters 
since ClientData is used internal-to-the-authnr in order to marshal 
info before hashing it.

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/117 using your GitHub account