Improve documentation and web test for web hooks

I wanted to share what I learned trying to debug web hooks using
netcat.

app/controllers/projects/hooks_controller.rb

@@ -28,7 +28,7 @@ class Projects::HooksController < Projects::ApplicationController
       status, message = TestHookService.new.execute(hook, current_user)
 
       if status
-        flash[:notice] = 'Hook successfully executed.'
+        flash[:notice] = "Hook successfully executed, HTTP #{status} #{message}"
       else
         flash[:alert] = "Hook execution failed: #{message}"
       end

app/models/hooks/web_hook.rb

@@ -59,7 +59,7 @@ class WebHook < ActiveRecord::Base
                               basic_auth: auth)
     end
 
-    [(response.code >= 200 && response.code < 300), ActionView::Base.full_sanitizer.sanitize(response.to_s)]
+    [response.code, ActionView::Base.full_sanitizer.sanitize(response.to_s)]
   rescue SocketError, OpenSSL::SSL::SSLError, Errno::ECONNRESET, Errno::ECONNREFUSED, Net::OpenTimeout => e
     logger.error("WebHook Error => #{e}")
     [false, e.to_s]

doc/web_hooks/web_hooks.md

@@ -13,6 +13,19 @@ You can configure webhooks to listen for specific events like pushes, issues or
 
 Webhooks can be used to update an external issue tracker, trigger CI builds, update a backup mirror, or even deploy to your production server.
 
+## Webhook endpoint tips
+
+If you are writing your own endpoint (web server) that will receive
+GitLab web hooks keep in mind the following things:
+
+-   Your endpoint should send its HTTP response as fast as possible. If
+    you wait too long, GitLab may decide the hook failed and retry it.
+-   Your endpoint should ALWAYS return a valid HTTP response. If you do
+    not do this then GitLab will think the hook failed and retry it.
+    Most HTTP libraries take care of this for you automatically but if
+    you are writing a low-level hook this is important to remember.
+-   GitLab ignores the HTTP status code returned by your endpoint.
+
 ## SSL Verification
 
 By default, the SSL certificate of the webhook endpoint is verified based on