Fix wrong constant type & other fixes

[54m]

Overview

See commit.

1. 58ea1fe Fix the wrong constant type ⚠️
2. 4e4cc43 Typo correction
3. b89478c Cleanup code
4. 53161b7 Consider nil ⚠️
5. f670bb4 Fixed to the correct tag name(omitemtpy -> omitempty) ⚠️

Other

Typo errors in public names have not been corrected.
I'd like to fix the public name as well ... :(

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@9047ed5). Click here to learn what that means.
The diff coverage is 68.00%.

@@            Coverage Diff            @@
##             master     #278   +/-   ##
=========================================
  Coverage          ?   77.57%           
=========================================
  Files             ?       36           
  Lines             ?     2288           
  Branches          ?        0           
=========================================
  Hits              ?     1775           
  Misses            ?      393           
  Partials          ?      120           

Impacted Files	Coverage Δ
linebot/client.go	68.00% <ø> (ø)
linebot/get_bot_info.go	100.00% <ø> (ø)
linebot/response.go	66.66% <ø> (ø)
linebot/richmenu.go	58.41% <ø> (ø)
linebot/message.go	81.73% <55.55%> (ø)
linebot/event.go	88.57% <100.00%> (ø)
linebot/oauth.go	86.20% <100.00%> (ø)
linebot/oauth2.go	86.66% <100.00%> (ø)
linebot/webhook.go	83.33% <100.00%> (ø)

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[kkdai]

LGTM