fix: Consider case of full and closed receiver

Handle case where the receiver heap first gets full, and all receivers
are dropped, in which case all senders will be notified about the
changed state, but they need to handle the case that no additional
capacity is available.

src/lib.rs

@@ -111,6 +111,10 @@ where
             let (guard, timed_out) = self.shared.senders.wait_timeout(guard, duration).unwrap();
             if timed_out.timed_out() {
                 return Err(SendError::Full(item));
+            } else if self.shared.is_closed() {
+                return Err(SendError::Closed(item));
+            } else if guard.is_full() {
+                return Err(SendError::Full(item));
             }
             guard
         };