Declare key in option_definition() for Views 3 export

I have not tested this, but your patch is changing the default value to be in the wrong format for the change in the data_source form item to a select, which just takes a string value as its default.

This is not true. The options returned by option definition array should be a keyed array where the keys refer to a variety of attributes of that option, one of which is its default value. Please see the base class views_object() from which all views handlers and plugins are derived:

  /**
   * Information about options for all kinds of purposes will be held here.
   * @code
   * 'option_name' => array(
   *  - 'default' => default value,
   *  - 'translatable' => TRUE/FALSE (wrap in t() on export if true),
   *  - 'contains' => array of items this contains, with its own defaults, etc.
   *      If contains is set, the default will be ignored and assumed to
   *      be array()
   *
   *  ),
   *  @endcode
   * ...
   */
  function option_definition() { return array(); }