Ruby mechanize post with header

I have page with js that post data via XMLHttpRequest and server side script check for this header, how to send this header?

agent = WWW::Mechanize.new { |a|
  a.user_agent_alias = 'Mac Safari'
  a.log = Logger.new('./site.log')
}

agent.post('http://site.com/board.php',
  {
    'act' => '_get_page',
    "gid" => 1,
    'order' => 0,
    'page' => 2
  }
) do |page|
  p page
end