use http method constant (#2155)

* use http method constant

* fix typo

binding/binding.go

@@ -84,7 +84,7 @@ var (
 // Default returns the appropriate Binding instance based on the HTTP method
 // and the content type.
 func Default(method, contentType string) Binding {
-	if method == "GET" {
+	if method == http.MethodGet {
 		return Form
 	}