[Fix] attachBody - reposition dropdown on message display

Resolves: select2#4614
Previous behavior: whenever a message is displayed, results are cleared
out, but dropdown is not repositioned, resulting in a floating search
box based off of last position.
Fixed behavior: when a message is displayed, reposition the dropdown
accordingly

src/js/select2/dropdown/attachBody.js

@@ -31,6 +31,11 @@ define([
           self._positionDropdown();
           self._resizeDropdown();
         });
+
+        container.on('results:message', function () {
+          self._positionDropdown();
+          self._resizeDropdown();
+        });
       }
     });