Fix 'fl' parameter default on PB, which should be to return all fields.
Fix 'fl' parameter default on PB, which should be to return all fields.

On HTTP, if no 'fl' option is given, the default is to return all
fields. In PB, if no 'fl' is given, it is represented as an empty
list rather than 'undefined', but we were previously interpreting
it as "ids only". This corrects that disparity.

basho/riak_search,wwezhuimeng/riak_search,wwezhuimeng/riak_search,xuejianhai/riak_search,xuejianhai/riak_search,basho/riak_search