Use unweighted mean of neighbors for complete_row to fix #413

This commit changes the behavior of complete_row.
It used a mean of neighbors weighted by the similarities with the query.
However, euclid_lsh returns a negated distance as a similarity.
It causes buggy results of complete_row.
So in #413, we decided to stop using weighting on complete_row, since in some cases as euclid_lsh, we cannot define an appropriate similarity for weighting.
This change is the fix for it.
Change complete_row test for checking unweighted averaging
Merge pull request #452 from jubatus/feature/fix_complete_row

complete_row of euclid lsh now uses the unweighted mean of query's neighbors. Fixes https://github.com/jubatus/jubatus/issues/413 .