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
 

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