HEX-1945: Fix typo in RebalanceDataSet (make the same as h2o-dev).
Note: This looks wrong to me, still.

Criterion for sparsity should be
sparse_len * 32 < dense_len, and not: sparse_len < 32 * dense_len (always true!)
Merge branch 'master' of https://github.com/0xdata/h2o
HEX-1945: Fix logic to determine whether the chunks are to be treated as sparse.
HEX-1945: Add asserts in sparse() call.
Merge branch 'master' of https://github.com/0xdata/h2o