Fix broken unit test

If the response is a bad request, we should probably not have anything
in the request message.

RestSharp.IntegrationTests/StatusCodeTests.cs

@@ -59,7 +59,7 @@ public void Handles_Different_Root_Element_On_Error()
 				var response = client.Execute<Response>(request);
 
 				Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
-				Assert.Equal("Not found!", response.Data.Message);
+				Assert.Null(response.Data);
 			}
 		}