Implement Series.combine_first

[itholic]

Implement Series.combine_first

• basic example

>>> s1 = ks.Series([1, np.nan])
>>> s2 = ks.Series([3, 4])
>>> s1.combine_first(s2)
0    1.0
1    4.0
Name: 0, dtype: float64

• MultiIndex

>>> midx1 = pd.MultiIndex([['lama', 'cow', 'falcon', 'koala'],
...                        ['speed', 'weight', 'length', 'power']],
...                       [[0, 3, 1, 1, 1, 2, 2, 2],
...                        [0, 2, 0, 3, 2, 0, 1, 3]])
>>> midx2 = pd.MultiIndex([['lama', 'cow', 'falcon'],
...                        ['speed', 'weight', 'length']],
...                       [[0, 0, 0, 1, 1, 1, 2, 2, 2],
...                        [0, 1, 2, 0, 1, 2, 0, 1, 2]])
>>> kser1 = ks.Series([45, 200, 1.2, 30, 250, 1.5, 320, 1], index=midx1)
>>> kser2 = ks.Series([-45, 200, -1.2, 30, -250, 1.5, 320, 1, -0.3], index=midx2)
>>> kser1
lama    speed      45.0
koala   length    200.0
cow     speed       1.2
        power      30.0
        length    250.0
falcon  speed       1.5
        weight    320.0
        power       1.0
Name: 0, dtype: float64
>>> kser2
lama    speed     -45.0
        weight    200.0
        length     -1.2
cow     speed      30.0
        weight   -250.0
        length      1.5
falcon  speed     320.0
        weight      1.0
        length     -0.3
Name: 0, dtype: float64

>>> kser1.combine_first(kser2)
cow     length    250.0
        power      30.0
        speed       1.2
        weight   -250.0
falcon  length     -0.3
        power       1.0
        speed       1.5
        weight    320.0
koala   length    200.0
lama    length     -1.2
        speed      45.0
        weight    200.0
Name: 0, dtype: float64

[itholic]

i considered putting Series.combine and Series.combine_first in single PR,

but their implementation concept was way different than i thought, so i separated them.

[codecov-io]

Codecov Report

Merging #1290 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1290      +/-   ##
==========================================
+ Coverage   95.25%   95.26%   +0.01%     
==========================================
  Files          34       34              
  Lines        7541     7559      +18     
==========================================
+ Hits         7183     7201      +18     
  Misses        358      358              

Impacted Files	Coverage Δ
databricks/koalas/missing/series.py	100.00% <ø> (ø)
databricks/koalas/series.py	96.86% <100.00%> (+0.07%)	⬆️

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[HyukjinKwon]

I think you should rebase and sync to the current master, @itholic .

[HyukjinKwon]

Looks fine otherwise.