adds failing test for hyperloglogplus sparse merge with intersection
Fixes ordering issue when merging sparse sets

The order of the sparse indexes is important, not the absolute order.
Otherwise, a smaller value (by absolute terms) can be inserted before a
smaller value (as far as sparse set index).
remove debugging print statement
Merge pull request #54 from derekgr/hllp-sparse-intersection-bug

HyperLogLogPlus sparse merge produces incorrect results on some sets