Add InitializeResponder to improve testing (palantir#34)

Allow code other than the default dispatcher to initialize a request
context for use with Set/GetResponder.

githubapp/dispatcher.go

@@ -134,10 +134,7 @@ func (d *eventDispatcher) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	ctx := r.Context()
 
 	// initialize context for SetResponder/GetResponder
-	// we store a pointer in the context so that functions deeper in the call
-	// tree can modify the value without creating a new context
-	var responder func(http.ResponseWriter, *http.Request)
-	ctx = context.WithValue(ctx, responderKey{}, &responder)
+	ctx = InitializeResponder(ctx)
 	r = r.WithContext(ctx)
 
 	eventType := r.Header.Get("X-GitHub-Event")
@@ -216,17 +213,25 @@ func DefaultResponseCallback(w http.ResponseWriter, r *http.Request, event strin
 
 type responderKey struct{}
 
+// InitializeResponder prepares the context to work with SetResponder and
+// GetResponder. It is used to test handlers that call SetResponder or to
+// implement custom event dispatchers that support responders.
+func InitializeResponder(ctx context.Context) context.Context {
+	var responder func(http.ResponseWriter, *http.Request)
+	return context.WithValue(ctx, responderKey{}, &responder)
+}
+
 // SetResponder sets a function that sends a response to GitHub after event
-// processing completes. This function may only be called from event handler
-// functions invoked by the event dispatcher.
+// processing completes. The context must be initialized by InitializeResponder.
+// The event dispatcher does this automatically before calling a handler.
 //
 // Customizing individual handler responses should be rare. Applications that
 // want to modify the standard responses should consider registering a response
 // callback before using this function.
 func SetResponder(ctx context.Context, responder func(http.ResponseWriter, *http.Request)) {
 	r, ok := ctx.Value(responderKey{}).(*func(http.ResponseWriter, *http.Request))
 	if !ok || r == nil {
-		panic("SetResponder() must be called from an event handler invoked by the go-githubapp event dispatcher")
+		panic("SetResponder() must be called with an initialized context, such as one from the event dispatcher")
 	}
 	*r = responder
 }