+htc add popular ContentType instances for convenience

akka-docs-dev/rst/java/code/docs/http/javadsl/server/HighLevelServerBindFailureExample.java

@@ -31,7 +31,7 @@ public class HighLevelServerBindFailureExample {
                 System.err.println("Something very bad happened! " + failure.getMessage());
                 system.shutdown();
             }
-        });
+        }, system.dispatcher());
 
         system.shutdown();
     }