title: Request is missing required `Host` header error in Scala Play 2.6.7 Akka
body: <p>In production mode, the server failed to start and keeps print the following log. I cannot access the web pages. But the code runs fine in development mode. How to resolve the problem?</p>
  
  <pre><code>2017-11-23T10:04:07+08 [INFO] from play.core.server.AkkaHttpServer [main]
          Listening for HTTP on /0:0:0:0:0:0:0:0:9000
  2017-11-23T10:04:08+08 [WARN] from akka.actor.ActorSystemImpl [application-akka.actor.default-dispatcher-4]
          Illegal request, responding with status '400 Bad Request': Request is 
          missing required `Host` header: Cannot establish effective URI of request 
          to `/`, request has a relative URI and is missing a `Host` header;
          consider setting `akka.http.server.default-host-header`
  
  2017-11-23T10:04:08+08 [WARN] from akka.actor.ActorSystemImpl [application-akka.actor.default-dispatcher-3]
          Illegal request, responding with status '400 Bad Request': Request is 
          missing required `Host` header: Cannot establish effective URI of request 
          to `/`, request has a relative URI and is missing a `Host` header;
          consider setting `akka.http.server.default-host-header`
  </code></pre>