bugfix: just hand over list of docs to sort function instead of tuple

In my opinion `riakc_pb_socket:search` does still not work with the latest patch, because `lists:sort` in `riak_solr_sort:sort` can't handle the tuple (DocsOrIDs) that is handed over as parameter. I am not sure, if my patch is the best way to handle it, at least now searching with sorting works for me and i thought: sorting of just a list of IDs can't work anyway. Thoughts?