Fix edge case for CX2Chunk

Very sparse column, that also all non-zero values are divisible by 10 so
a candidate for scaling down... which must happen to fit in a short.
Merge branch 'master' of github.com:0xdata/h2o
Clean up GBM/DRF enum building on the response col

Use the built-in values on Vec for isEnum & computing nclasses & making
enums.  Cleanup any generated enum column
Merge branch 'master' of github.com:0xdata/h2o