- Checks for the existence of the specified bucket type in
  riak_kv_pb_bucket:maybe_create_bucket_type/2 and
  riak_kv_pb_object:maybe_create_bucket_type/2 to properly handle
  requests for undefined bucket types (defect #875)
- Integrates feedback from @reiddraper in PR #966
   - Adds specs to riak_kv_pb_bucket:maybe_bucket_type_exists/1,
     riak_kv_pb_bucket:maybe_create_bucket_type/2, and
     riak_kv_pb_bucket:maybe_create_bucket_type/3
   - Simplifies the case statement in riak_kv_pb_bucket:process
Merge pull request #970 from basho/bugfix/jsb/fix-list-keys-missing-bucket-type-rebase

Verify bucket type existence for list keys and buckets operations

Reviewed-by: reiddraper