Add __bool__ function.

[ueshin]

Adding __bool__ function explicitly to raise an error when DataFrame, Series, and Index are used as bool, otherwise they might be used mistakenly.

[codecov-commenter]

Codecov Report

Merging #1526 into master will decrease coverage by 0.02%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #1526      +/-   ##
==========================================
- Coverage   94.02%   94.00%   -0.03%     
==========================================
  Files          36       36              
  Lines        8479     8484       +5     
==========================================
+ Hits         7972     7975       +3     
- Misses        507      509       +2     

Impacted Files	Coverage Δ
databricks/koalas/generic.py	96.67% <50.00%> (-0.35%)	⬇️
databricks/koalas/indexes.py	96.89% <50.00%> (-0.18%)	⬇️
databricks/koalas/utils.py	96.39% <100.00%> (+0.01%)	⬆️

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