Fix list_buckets in http backend

Since I upgraded riak-ruby-client to 1.4.1, I started getting the following error when calling client.list_buckets:

     ArgumentError:
       wrong number of arguments (1 for 0)
     # /riak-ruby-client/lib/riak/client/http_backend.rb:207:in `list_buckets'
     # /riak-ruby-client/lib/riak/client.rb:179:in `block in buckets'
     # /riak-ruby-client/lib/riak/client.rb:434:in `block in recover_from'
     # /riak-ruby-client/lib/riak/client.rb:432:in `recover_from'
     # /riak-ruby-client/lib/riak/client.rb:285:in `http'
     # /riak-ruby-client/lib/riak/client.rb:138:in `backend'
     # /riak-ruby-client/lib/riak/client.rb:178:in `buckets'

Seems like it's related to these recent changes: 40beeb74 and edf30352