Fix GroupBy.head to recognize agg_columns.

[ueshin]

Fixing GroupBy.head to recognize agg_columns.

>>> kdf = ks.DataFrame({"a": [1, 1, 1, 1, 2, 2, 2, 3, 3, 3] * 3, "b": [2, 3, 1, 4, 6, 9, 8, 10, 7, 5] * 3, "c": [3, 5, 2, 5, 1, 2, 6, 4, 3, 6] * 3})
>>> kdf.groupby("a")[["b"]].head(2).sort_index()
   a   b  c
0  1   2  3
1  1   3  5
4  2   6  1
5  2   9  2
7  3  10  4
8  3   7  3

This should be:

>>> pdf.groupby("a")[["b"]].head(2).sort_index()
    b
0   2
1   3
4   6
5   9
7  10
8   7

[codecov-io]

Codecov Report

Merging #1474 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1474      +/-   ##
==========================================
+ Coverage   93.74%   93.77%   +0.03%     
==========================================
  Files          36       36              
  Lines        8409     8420      +11     
==========================================
+ Hits         7883     7896      +13     
+ Misses        526      524       -2     

Impacted Files	Coverage Δ
databricks/koalas/groupby.py	88.85% <100.00%> (+0.21%)	⬆️
databricks/koalas/frame.py	95.58% <0.00%> (+0.04%)	⬆️
databricks/koalas/generic.py	97.02% <0.00%> (+0.37%)	⬆️

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[HyukjinKwon]

@ueshin, it looks good but can we have a private function for these cases? Looks like it's duplicated in #1472 https://github.com/databricks/koalas/pull/1473/files and #1471.

[ueshin]

Sure, I'll address it in the following PRs.

[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.