Added instance check for bucket_type argument to initializer.
Added instance check for bucket_type argument to initializer.

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