Xattr retrieval using ACCESS_DELETED flag returns error

Retrieval of xattrs for logically deleted documents (using SUBDOC_FLAG_ACCESS_DELETED) return a non-zero memd error code (0xcd) when successful:

https://github.com/couchbase/memcached/blob/0fba613ee677bc5ae8c94b248e01e0e0cbba738f/include/memcached/protocol_binary.h#L216

gocb doesn't recognize that error, terminates response processing, and returns 'an unknown error occured' error, and an empty DocumentFragment.

Based on local testing, things work if I let the 0xcd error through here:

https://github.com/couchbase/gocbcore/blob/master/agentops_subdoc.go#L272

Sample gocb code to reproduce the error:

https://gist.github.com/adamcfraser/1a8c35b9a741f1484f9db75b42eaaf86