widgetEventPrefix is lost when extending existing widget

Tested with dcb671539286dcc58b7e6ffa6035c647500e699a:

$.widget( "foo.bar", {
  widgetEventPrefix: "baz",

  ...
  _someMethod: function() {
    ...
    this._trigger( "event" );
    ...
  },
});
$.widget( "foo.bar", $.foo.bar, {
  ...
  _someOtherMethod: function() {
    ...
  }
  ...
});

When a widget is declared like this, the widgetEventPrefix is lost when the extension is applied. You can re-add it with an additional

$.widget( "foo.bar", $.foo.bar, {
  widgetEventPrefix: "baz"
});

after the declaration of the first extension. Nevertheless, the original should be preserved after the first extension is added, otherwise it's impossible to create optional extensions.

Here's a jsbin with a simple example: http://jsbin.com/uguxuw/4/edit