fix: Use POST method for Test Webhook endpoiont

[unnoy]

i got an error below when consuming TestWebhook endpoint.
"failed":"linebot: APIError 405 The request method, 'GET', is not supported

it seems only post method is allowed according to the doc.
https://developers.line.biz/en/reference/messaging-api/#test-webhook-endpoint

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (df06aa0) 78.93% compared to head (84f13cc) 78.93%.
Report is 1 commits behind head on master.

❗ Current head 84f13cc differs from pull request most recent head 283255b. Consider uploading reports for the commit 283255b to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #356   +/-   ##
=======================================
  Coverage   78.93%   78.93%           
=======================================
  Files          40       40           
  Lines        3712     3712           
=======================================
  Hits         2930     2930           
  Misses        590      590           
  Partials      192      192           

Files Changed	Coverage Δ
linebot/webhook.go	86.30% <100.00%> (ø)

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[Yang-33]

Thank you!

[kkdai]

LGTM