Fix MR bucket type incompatibility

Follow https://github.com/basho/riak_kv/pull/703
and list bucket type as the fourth element when input is a list
([Type, Bucket, Key, KeyData] -> [Bucket, Key, KeyData, Type]).
This should fix the http_bucket_types riak test
Merge pull request #26 from basho/bugfix/bucket-types-mr-incompatibility

Fix MR bucket type incompatibility