Make Series.astype(bool) follow the concept of "truthy" and "falsey".

[ueshin]

Making Series.astype(bool) follow the concept of "truthy" and "falsey".

>>> kser = ks.Series(["hi", "hi ", " ", " \t", "", None], name="x")
>>> kser
0      hi
1     hi
2
3      \t
4
5    None
Name: x, dtype: object
>>> kser.astype(bool)
0     True
1     True
2     True
3     True
4    False
5    False
Name: x, dtype: bool
>>> kser.str.strip().astype(bool)
0     True
1     True
2    False
3    False
4    False
5    False
Name: x, dtype: bool

Resolves #1430.

[codecov-io]

Codecov Report

Merging #1431 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1431   +/-   ##
=======================================
  Coverage   95.14%   95.14%           
=======================================
  Files          34       34           
  Lines        7967     7974    +7     
=======================================
+ Hits         7580     7587    +7     
  Misses        387      387           

Impacted Files	Coverage Δ
databricks/koalas/series.py	96.95% <100.00%> (+0.02%)	⬆️
databricks/koalas/typedef.py	82.84% <100.00%> (ø)

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