AngularJS - Can't pass a header on a $http request - angularjs

Question

I'm trying to do requests to a RestAPI coded with Codeigniter 3. Access-Control-Allow-Origin and Access-Control-Allow-Headers are set to "*". The API has been successfully tested on Postman.
I'm coding a Web App using AngularJS, and trying to do requests to that API. The following code works properly:
api_service.getUsers = function() {
return $http({
method : 'GET',
url : url + '?dd-api-key=' + api_key
});
}
As this other code functions well too:
api_service.getUsers = function() {
return $http.get(url + '?dd-api-key=' + api_key);
}
But I need to send the API key as a header and not in the URL. So I wrote the function this way:
api_service.getUsers = function() {
return $http({
method: 'GET',
url: url,
headers: {
'accept': undefined,
'dd-api-key': api_key
}
});
}
That code doesn't work. Google Chrome console displays me this:
OPTIONS http://www.myweb.com/api/v1/users
XMLHttpRequest cannot load http://www.myweb.com/api/v1/users. Invalid HTTP status code 403
And at the network tab it shows that the request uses a method type 'OPTIONS', not a 'GET' method.
The Response Headers are:
Access-Control-Allow-Headers:*
Access-Control-Allow-Methods:PUT, GET, POST, DELETE, OPTIONS
Access-Control-Allow-Origin:*
Cache-Control:no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Connection:keep-alive
Content-Language:es-ES
Content-Length:45
Content-Type:application/json; charset=utf-8
Date:Wed, 25 Mar 2015 01:45:09 GMT
Expires:Thu, 19 Nov 1981 08:52:00 GMT
Pragma:no-cache
Server:nginx/1.6.2
And the Request Headers are:
Accept:*/*
Accept-Encoding:gzip, deflate, sdch
Accept-Language:es-ES,es;q=0.8,en;q=0.6
Access-Control-Request-Headers:dd-api-key
Access-Control-Request-Method:GET
Connection:keep-alive
Host:www.myweb.com
Origin:http://127.0.0.1:55652
Referer:http://127.0.0.1:55652/index.html
User-Agent:Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2272.101 Safari/537.36
Excuse me if my english is not correct.
Many thanks.

Answer 1

you should add header with this:
$http.defaults.headers.common.dd-api-key = api_key;
You can add it in some config as global.