fix(SenderLink): delete unsettled send on disposition

Setting the value to undefined maintains the property on the
_unsettledSends object.

lib/sender_link.js

@@ -233,7 +233,7 @@ SenderLink.prototype._dispositionReceived = function(details) {
     }
 
     this._unsettledSends[messageId](err, details.state);
-    this._unsettledSends[messageId] = undefined;
+    delete this._unsettledSends[messageId];
   }
 };