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
ajax_headers = { 'X-Requested-With' => 'XMLHttpRequest', 'Content-Type' => 'application/json; charset=utf-8', 'Accept' => 'application/json, text/javascript, */*'}
params = {'emailAddress' => 'me@my.com'}.to_json
response = agent.post( 'http://example.com/login', params, ajax_headers)
The above code works for me (Mechanize 1.0) as a way to make the server think the request is coming via AJAX, but as stated in other answers it depends what the server is looking for, it will be different for different frameworks/js library combos.
The best thing to do is use Firefox HTTPLiveHeaders plugin or HTTPScoop and look at the request headers sent by the browser and just try and replicate that.