Replace http constants with stdlib ones, i.e.: http.MethodGet instead of echo.GET (#1205)

middleware/cors.go

@@ -52,7 +52,7 @@ var (
 	DefaultCORSConfig = CORSConfig{
 		Skipper:      DefaultSkipper,
 		AllowOrigins: []string{"*"},
-		AllowMethods: []string{echo.GET, echo.HEAD, echo.PUT, echo.PATCH, echo.POST, echo.DELETE},
+		AllowMethods: []string{http.MethodGet, http.MethodHead, http.MethodPut, http.MethodPatch, http.MethodPost, http.MethodDelete},
 	}
 )
 
@@ -105,7 +105,7 @@ func CORSWithConfig(config CORSConfig) echo.MiddlewareFunc {
 			}
 
 			// Simple request
-			if req.Method != echo.OPTIONS {
+			if req.Method != http.MethodOptions {
 				res.Header().Add(echo.HeaderVary, echo.HeaderOrigin)
 				res.Header().Set(echo.HeaderAccessControlAllowOrigin, allowOrigin)
 				if config.AllowCredentials {