Implement IOSHttpRequest and IOSHttpResponse http interfaces and use instead of OSHttpRequest/OSHttpResponse.

This is required for the substitution of different HTTP servers or the newer HttpServer.dll without having to commit to a particular implementation.
This is also required to write regression tests that involve the HTTP layer.
If you need to recompile, all you need to do is replace OSHttpRequest/OSHttpResponse references with IOSHttpRequest/IOSHttpResponse.

OpenSim/Framework/Servers/HttpServer/RestMethod.cs

@@ -28,5 +28,5 @@
 namespace OpenSim.Framework.Servers.HttpServer
 {
     public delegate string RestMethod(string request, string path, string param,
-                                      OSHttpRequest httpRequest, OSHttpResponse httpResponse);
+                                      IOSHttpRequest httpRequest, IOSHttpResponse httpResponse);
 }