Tweak msg_send! so return type cannot be omitted

Previously, the structure of the macro meant that the return type was
inferred to () when no type was specified. This relied on an edge case
in the compiler and will produce undefined behavior in the future.

This fixes #62.

src/macros.rs

@@ -111,30 +111,38 @@ let _: () = msg_send![obj, setArg1:1 arg2:2];
 macro_rules! msg_send {
     (super($obj:expr, $superclass:expr), $name:ident) => ({
         let sel = sel!($name);
+        let result;
         match $crate::__send_super_message(&*$obj, $superclass, sel, ()) {
             Err(s) => panic!("{}", s),
-            Ok(r) => r,
+            Ok(r) => result = r,
         }
+        result
     });
     (super($obj:expr, $superclass:expr), $($name:ident : $arg:expr)+) => ({
         let sel = sel!($($name:)+);
+        let result;
         match $crate::__send_super_message(&*$obj, $superclass, sel, ($($arg,)*)) {
             Err(s) => panic!("{}", s),
-            Ok(r) => r,
+            Ok(r) => result = r,
         }
+        result
     });
     ($obj:expr, $name:ident) => ({
         let sel = sel!($name);
+        let result;
         match $crate::__send_message(&*$obj, sel, ()) {
             Err(s) => panic!("{}", s),
-            Ok(r) => r,
+            Ok(r) => result = r,
         }
+        result
     });
     ($obj:expr, $($name:ident : $arg:expr)+) => ({
         let sel = sel!($($name:)+);
+        let result;
         match $crate::__send_message(&*$obj, sel, ($($arg,)*)) {
             Err(s) => panic!("{}", s),
-            Ok(r) => r,
+            Ok(r) => result = r,
         }
+        result
     });
 }