Replace 200, HEAD and OPTIONS with constants from net/http (#243)

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## What type of PR is this? (check all applicable)

- [x] Refactor
- [ ] Feature
- [ ] Bug Fix
- [ ] Optimization
- [ ] Documentation Update

## Description

A previous commit (#241) replaced most HTTP constants with those from
standard library `net/http`, but some was missed. This PR catches 3 of
them.

## Related Tickets & Documents

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- Related Issue: None
- Closes: None

## Added/updated tests?

- [ ] Yes
- [x] No, and this is why: Trivial changes, tests not required
- [ ] I need help with writing tests

## Run verifications and test

- [ ] `make verify` is passing (which I believe is not my fault)
- [x] `make test` is passing

cors.go

@@ -26,14 +26,14 @@ type cors struct {
 type OriginValidator func(string) bool
 
 var (
-	defaultCorsOptionStatusCode = 200
-	defaultCorsMethods          = []string{http.MethodGet, "HEAD", http.MethodPost}
+	defaultCorsOptionStatusCode = http.StatusOK
+	defaultCorsMethods          = []string{http.MethodGet, http.MethodHead, http.MethodPost}
 	defaultCorsHeaders          = []string{"Accept", "Accept-Language", "Content-Language", "Origin"}
 	// (WebKit/Safari v9 sends the Origin header by default in AJAX requests).
 )
 
 const (
-	corsOptionMethod           string = "OPTIONS"
+	corsOptionMethod           string = http.MethodOptions
 	corsAllowOriginHeader      string = "Access-Control-Allow-Origin"
 	corsExposeHeadersHeader    string = "Access-Control-Expose-Headers"
 	corsMaxAgeHeader           string = "Access-Control-Max-Age"