Test the size of every struct defined here.

src/delay.rs

@@ -39,6 +39,11 @@ mod tests {
     use error::Error;
     use std::str::FromStr;
 
+    #[test]
+    fn test_size() {
+        assert_size!(Delay, 112);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<delay xmlns='urn:xmpp:delay' from='capulet.com' stamp='2002-09-10T23:08:25Z'/>".parse().unwrap();