I am assuming _helper.server.http is a different function, I think you are making a mistake while passing the header
You have to send the header as a object
var customHeader = {'X-user': 'pqrs'}
to
var hello = _helper.server.http($http, url, customHeader)
and under the function http in _helper.server, you have to add the headers there and then call the http post method,
http: function($http, urlParam, param) {
var request = {
url: urlParam,
headers: param
};
return $http.post(request);
}
It would help if you post more code, this could be the mistake that you are making