Make httpbin.HTTPBin implement http.Handler interface (#100)

This is a very small improvement to developer ergonomics.

README.md

@@ -62,7 +62,7 @@ import (
 
 func TestSlowResponse(t *testing.T) {
 	app := httpbin.New()
-	testServer := httptest.NewServer(app.Handler())
+	testServer := httptest.NewServer(app)
 	defer testServer.Close()
 
 	client := http.Client{

example_test.go

@@ -12,7 +12,7 @@ import (
 
 func TestSlowResponse(t *testing.T) {
 	app := httpbin.New()
-	testServer := httptest.NewServer(app.Handler())
+	testServer := httptest.NewServer(app)
 	defer testServer.Close()
 
 	client := http.Client{

examples/custom-instrumentation/main.go

@@ -13,12 +13,12 @@ import (
 func main() {
 	statsdClient, _ := statsd.New("")
 
-	h := httpbin.New(
+	app := httpbin.New(
 		httpbin.WithObserver(datadogObserver(statsdClient)),
 	)
 
 	listenAddr := "0.0.0.0:8080"
-	http.ListenAndServe(listenAddr, h.Handler())
+	http.ListenAndServe(listenAddr, app)
 }
 
 func datadogObserver(client statsd.ClientInterface) httpbin.Observer {

httpbin/httpbin.go

@@ -106,6 +106,9 @@ type HTTPBin struct {
 
 	// The hostname to expose via /hostname.
 	hostname string
+
+	// The app's http handler
+	handler http.Handler
 }
 
 // DefaultParams defines default parameter values
@@ -236,9 +239,18 @@ func New(opts ...OptionFunc) *HTTPBin {
 	for _, opt := range opts {
 		opt(h)
 	}
+	h.handler = h.Handler()
 	return h
 }
 
+// ServeHTTP implememnts the http.Handler interface.
+func (h *HTTPBin) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	h.handler.ServeHTTP(w, r)
+}
+
+// Assert that HTTPBin implements http.Handler interface
+var _ http.Handler = &HTTPBin{}
+
 // OptionFunc uses the "functional options" pattern to customize an HTTPBin
 // instance
 type OptionFunc func(*HTTPBin)