ControllerListVolumes - Define the initial value for starting_token

[Jgoldick]

The spec does not indicate whether a new enumeration should just omit this field or set it to an empty string.

[jdef]

There's some ambiguity in the spec here with regard to unset proto3 "primitive" fields. Apparently it can be hard to tell the difference between an unset primitive and "zero-value" of the field. There are several places in the spec where we need to tighten this up. See #22

[saad-ali]

This has been resolved.