add example for simple http.Handler wrapper

_examples/convert-handlers/nethttp/wrapper/main.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/kataras/iris/v12"
+)
+
+func main() {
+	app := iris.New()
+
+	httpThirdPartyWrapper := StandardWrapper(Options{
+		Message: "test_value",
+	})
+
+	// This case
+	app.WrapRouter(func(w http.ResponseWriter, r *http.Request, router http.HandlerFunc) {
+		httpThirdPartyWrapper(router).ServeHTTP(w, r)
+		// If was func(http.HandlerFunc) http.HandlerFunc:
+		// httpThirdPartyWrapper(router.ServeHTTP).ServeHTTP(w, r)
+	})
+
+	app.Get("/", index)
+	app.Listen(":8080")
+}
+
+func index(ctx iris.Context) {
+	ctx.Writef("Message: %s\n", ctx.Value(msgContextKey))
+}
+
+type Options struct {
+	Message string
+}
+
+type contextKey uint8
+
+var (
+	msgContextKey contextKey = 1
+)
+
+func StandardWrapper(opts Options) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			// ...
+			req := r.WithContext(context.WithValue(r.Context(), msgContextKey, opts.Message))
+			next.ServeHTTP(w, req)
+		})
+	}
+}