Update expected return to match change in riak_pb API

See https://github.com/basho/riak-erlang-client/commit/2b68a97710efa0167ab1caeca653eaa9d3a8747e
for details. Basically, a counter with no local modifications has `undefined` as the 3rd element in the tuple, not `0`. This PR addresses the change.

To test? Run the test.