OwinFeature collection throws NotSupported exception as the featurecollection is immutable

Addresses: aspnet/HttpAbstractions#207

Author: Praburaj

src/Microsoft.AspNet.Hosting/Builder/HttpContextFactory.cs

@@ -11,7 +11,7 @@ public class HttpContextFactory : IHttpContextFactory
     {
         public HttpContext CreateHttpContext(IFeatureCollection featureCollection)
         {
-            return new DefaultHttpContext(featureCollection);
+            return new DefaultHttpContext(new FeatureCollection(featureCollection));
         }
     }
 }