Fix GroupBy.apply to follow complex group keys.

[ueshin]

Fixing GroupBy.apply to follow complex group keys.

>>> kdf = ks.DataFrame({"a": [1, 2, 3, 4, 5, 6], "b": [1, 1, 2, 3, 5, 8], "c": [1, 4, 9, 16, 25, 36]})
>>> kdf.groupby(kdf.b // 5).apply(lambda x: x + x.min())
    a   b   c
0   2   2   2
1   3   2   5
2   6   4  18
3   8   6  32
4  10  10  50
5  12  16  72

This should be:

>>> pdf.groupby(pdf.b // 5).apply(lambda x: x + x.min())
    a   b   c
0   2   2   2
1   3   2   5
2   4   3  10
3   5   4  17
4  10  10  50
5  11  13  61

[HyukjinKwon]

LGTM except https://github.com/databricks/koalas/pull/1474/files#r422865501

[ueshin]

Thanks! I'd merge this now. I'll address the comment in the following PRs.