Test the size of every struct defined here.

src/message_correct.rs

@@ -25,6 +25,11 @@ mod tests {
     use minidom::Element;
     use error::Error;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Replace, 24);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<replace xmlns='urn:xmpp:message-correct:0' id='coucou'/>".parse().unwrap();