Adpat http middleware handler to filter

[ggicci]

Let's treat the following signature as an "http middleware handler":

type HttpMiddlewareHandler func(http.Handler) http.Handler

I believe many of us are implementing middleware functions in this way. It's really easy to chain these kind of handlers together to work as one complete http handler. And there are already many middleware functions in the community implemented this way. Here are some packages or frameworks supporting this:

1. https://github.com/justinas/alice
2. https://github.com/gorilla/handlers
3. https://github.com/go-chi/chi/tree/master/middleware
4. ...

In go-restful, we can find an easy way to turn such a middleware handler to a filter. It should work like a charm.

Here's my proposal of the API:

// Adapts a middleware handler to a filter.
func HttpMiddlewareHandlerToFilter(middleware HttpMiddlewareHandler) FilterFunction { ... }

// Usage Demo
//
// Sample middleware handler, for authentication.
func auth(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.Header.Get("Authorization") != "admin" {
			http.Error(w, http.StatusText(http.StatusUnauthorized), http.StatusUnauthorized)
			return
		}

		next.ServeHTTP(w, r)
	})
}

ws := new(WebService)
ws.Route(ws.GET("/users").Filter(
	HttpMiddlewareHandlerToFilter(auth),
).To(listUsers))

[ggicci]

Similar discussion from iris community: kataras/iris#1885