NO-ISSUE Code refactoring

[linxGnu]

This PR makes changes:

• The sdk is not responsible for handling context (in each http request) error. Therefore, following code will be removed:

                res, err := client.httpClient.Do(req.WithContext(ctx))
		if err != nil {
			select {
			case <-ctx.Done():
				err = ctx.Err()
			default:
			}
		}

• Using constant defined in http std package instead of hard code. In this way:
  req, err := http.NewRequest("GET", client.url(endpoint), nil) will be rewritten as
  req, err := http.NewRequest(http.MethodGet, client.url(endpoint), nil)

• Update test along with changes

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #150 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
- Coverage   72.11%   72.07%   -0.04%     
==========================================
  Files          23       23              
  Lines        1757     1751       -6     
==========================================
- Hits         1267     1262       -5     
+ Misses        413      412       -1     
  Partials       77       77

Impacted Files	Coverage Δ
linebot/client.go	73.52% <100%> (-0.8%)	⬇️

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[nasa9084]

LGTM