Tidy codes and add tests

[rinchsan]

I tidied some codes and add more tests mainly to api request.

[codecov]

Codecov Report

Merging #13 (9fa0fa0) into master (6ab2ab8) will increase coverage by 6.44%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   88.07%   94.52%   +6.44%     
==========================================
  Files           9        9              
  Lines         151      146       -5     
==========================================
+ Hits          133      138       +5     
+ Misses         12        4       -8     
+ Partials        6        4       -2     

Impacted Files	Coverage Δ
query.go	86.66% <80.00%> (+10.80%)	⬆️
error.go	100.00% <100.00%> (+13.63%)	⬆️
jwt.go	100.00% <100.00%> (+16.66%)	⬆️
update.go	89.47% <0.00%> (+5.26%)	⬆️
validate.go	88.23% <0.00%> (+5.88%)	⬆️

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