#280 Add headers to response (#286)

* #280 can now add response headers inc trace id, now need to consolidate the header place holder stuff

* #280 changed port for linux tests

* #280 lots of hacking around to handle errors and consolidate placeholders into one class

test/Ocelot.AcceptanceTests/Steps.cs

@@ -318,6 +318,12 @@ namespace Ocelot.AcceptanceTests
             header.First().ShouldBe(value);
         }
 
+        public void ThenTheTraceHeaderIsSet(string key)
+        {
+            var header = _response.Headers.GetValues(key);
+            header.First().ShouldNotBeNullOrEmpty();
+        }
+
         public void GivenOcelotIsRunningUsingJsonSerializedCache()
         {
             _webHostBuilder = new WebHostBuilder();