Added postcondition to ensure the result of {HTTP_CLIENT_REQUEST}.response is attached.

(useless with void-safety compilation, but keep it for non void-safe execution).

library/network/http_client/src/http_client_request.e

@@ -57,6 +57,8 @@ feature -- Access
 			-- Execute the request and return the response.
 			-- note: two consecutive calls will trigger two executions!
 		deferred
+		ensure
+			Result_set: Result /= Void
 		end
 
 feature {HTTP_CLIENT_SESSION} -- Execution