title: Rails server hangs (sometimes) after two requests by the frontend
body: <p>I'm building a single page application. My frontend (written in React), makes two calls to the backend and the rails server hangs. I mean, the execution doesn't reach the actions involved and nothing happens. Suggestions? I spent so much time on this and I don't know what to think anymore. This is the console's output.</p>
  
  <p>I tried with webrick as well so I don't think the issue is from Puma.</p>
  
  <pre><code>=&gt; Booting Puma
  =&gt; Rails 5.1.3 application starting in development on http://localhost:3002
  =&gt; Run `rails server -h` for more startup options
  Puma starting in single mode...
  * Version 3.10.0 (ruby 2.4.1-p111), codename: Russell's Teapot
  * Min threads: 5, max threads: 5
  * Environment: development
  * Listening on tcp://localhost:3002
  Use Ctrl-C to stop
  Started GET "/api/working-groups" for ::1 at 2017-09-05 11:29:52 +0200
  Started GET "/api/invoicing-sessions" for ::1 at 2017-09-05 11:29:52 +0200
  </code></pre>