Fix test expectation, after index query changes return term in Key's place

Failing query test case:

    V1 = backend_eqc:init_backend(riak_kv_eleveldb_backend,false, [{data_root,"test/eleveldb-backend"}]).
    riak_kv_eleveldb_backend:put(<<"b2">>,<<"k1">>, [{add,<<"k_int">>,0}], <<110>>, V1).
    riak_kv_eleveldb_backend:fold_keys(fun(B, K, Acc) -> [{B, K}|Acc] end, [],

Return value:

    [{index,<<"b2">>,{range,<<"k_int">>,0,0}}], V1).

@rdb Please check to see how much effort vs. value might be required
to address the TODO comment.  If it's too much work now, then please
create an issue for fixing & removing that TODO item so that the matter
doesn't get lost.

After this commit, tested via:

    P = backend_eqc : property ( riak_kv_eleveldb_backend , false , [ { data_root , "test/eleveldb-backend" } ] ).
    eqc:quickcheck(eqc:testing_time(4*300, P)).
    [...]
    OK, passed 9670 tests