Chrome Extension: how to change origin in AJAX request header?

I'm trying to manually set an origin in an ajax request header. In my background.js, I have this

var ajaxResponse;
$.ajax({
    type:'POST',
    url:'www.somewebsite.com/login/login.asp',
    headers:{
            'origin': 'https://www.somewebsite.com'
    },
    success: function(response){
        ajaxResponse = response;
    }
});

As you can see, the origin is changed. But when this Chrome extension get executed, the origin gets override to chrome-extension://iphajdjhoofhlpldiilkujgommcolacc and the console gives error 'Refused to set unsafe header "origin"'

I've followed Chrome API (http://developer.chrome.com/extensions/xhr.html), and already set the permission as follows

"permissions": [
     "https://www.somewebsite.com/*"
 ],

Does anyone know how to properly set the origin in header? Thanks!