Add assertions for counters feature detection.
Add assertions for counters feature detection.

basho/riak-python-client,GabrielNicolasAvellaneda/riak-python-client,basho/riak-python-client,basho/riak-python-client,bmess/riak-python-client,GabrielNicolasAvellaneda/riak-python-client,bmess/riak-python-client