Create authentication middleware example. (#340)

* Create authentication middleware example.

For #339

* Fix example test filename.

example_authentication_middleware_test.go

@@ -0,0 +1,46 @@
+package mux_test
+
+import (
+	"log"
+	"net/http"
+
+	"github.com/gorilla/mux"
+)
+
+// Define our struct
+type authenticationMiddleware struct {
+	tokenUsers map[string]string
+}
+
+// Initialize it somewhere
+func (amw *authenticationMiddleware) Populate() {
+	amw.tokenUsers["00000000"] = "user0"
+	amw.tokenUsers["aaaaaaaa"] = "userA"
+	amw.tokenUsers["05f717e5"] = "randomUser"
+	amw.tokenUsers["deadbeef"] = "user0"
+}
+
+// Middleware function, which will be called for each request
+func (amw *authenticationMiddleware) Middleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		token := r.Header.Get("X-Session-Token")
+
+		if user, found := amw.tokenUsers[token]; found {
+			// We found the token in our map
+			log.Printf("Authenticated user %s\n", user)
+			next.ServeHTTP(w, r)
+		} else {
+			http.Error(w, "Forbidden", 403)
+		}
+	})
+}
+
+func Example_authenticationMiddleware() {
+	r := mux.NewRouter()
+	r.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		// Do something here
+	})
+	amw := authenticationMiddleware{}
+	amw.Populate()
+	r.Use(amw.Middleware)
+}