split_into_nhot() doesn't work properly on views

[oleksiyskononenko]

split_into_nhot() works just fine on the entire frame and views that start from zero row

>>> import datatable as dt
>>> from datatable import split_into_nhot
>>> df = dt.Frame(["a", "b"])
>>> split_into_nhot(df)
      a   b
---  --  --
 0    1   0
 1    0   1
>>> split_into_nhot(df[0,:])
      a
---  --
 0    1

If a view doesn't start at the zero row, it returns wrong results

>>> split_into_nhot(df[1,:])
      a
---  --
 0    1

Expected output in this case is

>>> split_into_nhot(df[1,:])
      b
---  --
 0    1

Need this to be fixed to finalize #1499.