Fix window functions to resolve.

[ueshin]

After the window functions, we should always resolve the InternalFrame; otherwise the window will be applied later, causing a weird behavior:

>>> kdf = ks.DataFrame({"Col1": [10, 20, 15, 30, 45], "Col2": [13, 23, 18, 33, 48], "Col3": [17, 27, 22, 37, 52]})
>>> kdf
   Col1  Col2  Col3
0    10    13    17
1    20    23    27
2    15    18    22
3    30    33    37
4    45    48    52
>>> kdf['Col1'].shift().loc[kdf['Col1'] == 20]
1   NaN
Name: Col1, dtype: float64

This should be:

>>> pdf = kdf.to_pandas()
>>> pdf['Col1'].shift().loc[pdf['Col1'] == 20]
1    10.0
Name: Col1, dtype: float64

Resolves #2078.

[codecov-io]

Codecov Report

Merging #2090 (20120e3) into master (becd789) will decrease coverage by 0.01%.
The diff coverage is 95.45%.

@@            Coverage Diff             @@
##           master    #2090      +/-   ##
==========================================
- Coverage   94.55%   94.53%   -0.02%     
==========================================
  Files          57       57              
  Lines       13227    13236       +9     
==========================================
+ Hits        12507    12513       +6     
- Misses        720      723       +3     

Impacted Files	Coverage Δ
databricks/koalas/groupby.py	91.63% <ø> (ø)
databricks/koalas/spark/accessors.py	94.64% <90.00%> (-0.33%)	⬇️
databricks/koalas/base.py	97.53% <100.00%> (ø)
databricks/koalas/frame.py	96.61% <100.00%> (ø)
databricks/koalas/series.py	96.78% <100.00%> (ø)
databricks/conftest.py	93.75% <0.00%> (-3.13%)	⬇️

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