datastore: use iterator.Done
datastore: use iterator.Done

Change-Id: I3767746033e4e9a1bf1dc12e3e52b5a9133f9c32
Reviewed-on: https://code-review.googlesource.com/7717
Reviewed-by: Dave Day <4cba7a21e1f1fc896442779dcb55aeda6df97dfe@golang.org>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go