Fix update and add tests for update and join.

[ueshin]

No description provided.

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Softagram Impact Report for pull/848 (head commit: 85d257e)

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[codecov-io]

Codecov Report

Merging #848 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #848      +/-   ##
==========================================
+ Coverage   94.17%   94.17%   +<.01%     
==========================================
  Files          32       32              
  Lines        6007     6008       +1     
==========================================
+ Hits         5657     5658       +1     
  Misses        350      350

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.09% <100%> (ø)	⬆️

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[HyukjinKwon]

Hmmm .I might have to add a retry logic at:

  File "/home/travis/build/databricks/koalas/docs/source/../../dev/gendoc.py", line 195, in download_pandoc_if_needed
    pandoc_download.download_pandoc(targetfolder=path)
  File "/home/travis/miniconda/envs/test-environment/lib/python3.6/site-packages/pypandoc/pandoc_download.py", line 190, in download_pandoc
...
    raise URLError(err)
urllib.error.URLError: <urlopen error [Errno 110] Connection timed out>
* Downloading pandoc from https://github.com/jgm/pandoc/releases/download/2.7.3/pandoc-2.7.3-1-amd64.deb ...
Makefile:21: recipe for target 'html' failed

retriggering..

[ueshin]

Thanks! merging.