Code refinement

[chfw]

With your PR, here is a check list:

• [ ] Has Test cases written
• [ ] Has all code lines tested
• [ ] Has make format been run?
• [ ] Please update CHANGELOG.yml(not CHANGELOG.rst)
• [ ] Passes all Travis CI builds
• [ ] Has fair amount of documentation if your change is complex
• [ ] run 'make format' so as to confirm the pyexcel organisation's coding style
• [ ] Please add yourself to 'contributors' section of pyexcel-io.yml (if not found, please use CONTRIBUTORS.rst)
• [ ] Agree on NEW BSD License for your contribution

[codecov-commenter]

Codecov Report

Merging #84 into dev will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev      #84      +/-   ##
==========================================
- Coverage   96.96%   96.96%   -0.01%     
==========================================
  Files          50       50              
  Lines        3226     3225       -1     
==========================================
- Hits         3128     3127       -1     
  Misses         98       98              

Impacted Files	Coverage Δ
pyexcel_io/_compact.py	62.96% <ø> (-1.33%)	:arrow_down:
pyexcel_io/constants.py	100.00% <ø> (ø)
pyexcel_io/io.py	98.23% <ø> (ø)
pyexcel_io/database/__init__.py	100.00% <100.00%> (ø)
pyexcel_io/database/querysets.py	95.12% <100.00%> (ø)
pyexcel_io/plugin_api/abstract_sheet.py	100.00% <100.00%> (ø)
pyexcel_io/plugins.py	96.29% <100.00%> (ø)
pyexcel_io/readers/__init__.py	100.00% <100.00%> (ø)
pyexcel_io/readers/csv_sheet.py	98.14% <100.00%> (-0.02%)	:arrow_down:
pyexcel_io/writers/__init__.py	100.00% <100.00%> (ø)
... and 1 more

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