flag_handler_filter_flagged::option_definition breaks views exposed forms when AJAX is enabled

Problem/Motivation

Steps to reproduce:

1. Create a view with a flagged exposed filter.
2. Enable AJAX.
3. When navigating using the pager, the options value will always switch to '1'.

The following code seems to kick in:

  function option_definition() {
    $options = parent::option_definition();
    $options['value'] = array('default' => 1);
    return $options;
  }

When this code is removed, all seems to work as expected.

Proposed resolution

I don't understand why this exists, perhaps it can simply be removed?

Remaining tasks

User interface changes

API changes

Data model changes