Feature/#623 (#632)

* #630 only set status code if response hasnt started, otherwise exception

* #623 made {RemoteIpAddress} available as placeholder so you can do x-forwarded-for

* #623 local address different on mac, windows and linux for integration test

test/Ocelot.AcceptanceTests/HeaderTests.cs

@@ -4,6 +4,7 @@ namespace Ocelot.AcceptanceTests
     using System.Collections.Generic;
     using System.Linq;
     using System.Net;
+    using System.Net.Sockets;
     using System.Threading.Tasks;
     using Microsoft.AspNetCore.Http;
     using Ocelot.Configuration.File;