Test the size of every struct defined here.

src/jingle_message.rs

@@ -121,6 +121,11 @@ impl From<JingleMI> for Element {
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_size() {
+        assert_size!(JingleMI, 136);
+    }
+
     #[test]
     fn test_simple() {
         let elem: Element = "<accept xmlns='urn:xmpp:jingle-message:0' id='coucou'/>".parse().unwrap();