/**
 * DoPost, just does a post call
 * and passes the response to a 
 * callback
 * 
 * @param {string} url 
 * @param {Object} json 
 * @param {Function} callback 
 * @param {Array} headers 
 */
function doPost(url, json, callback, headers){

    var xhr = new XMLHttpRequest();
    xhr.open("POST", url, true);

    //Send the proper header information along with the request
    xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");

    if(typeof headers != 'undefined')
        for(var header in headers)
            xhr.setRequestHeader(header, headers[header]);
    
    xhr.onreadystatechange = function() { // Call a function when the state changes.
        if (this.readyState === XMLHttpRequest.DONE && this.status === 200) {
            console.log('____response___',this)
            
            console.log('this.responseText',this.responseText)
                    
            var json = null;
            if(this.responseText)
                try { 
                    json = JSON.parse(this.responseText);
                } catch(err) {
                    json = { response: this.responseText };
                }
                callback(json);
        }
        
        if (this.readyState === XMLHttpRequest.DONE && this.status != 200) {
            console.log('____response___',this)
            
            console.log('this.responseText',this.responseText)
                    
            var json = null;
            if(this.responseText)
                try { 
                    json = JSON.parse(this.responseText);
                } catch(err) {
                    json = { response: this.responseText };
                }
                callback(json);
        }

        console.log('this.readyState',this.readyState, this.status, this)
    }
    
    var data = encodeURI(Object.keys(json).map(function(k) { return k+'='+json[k]; }).join('&'));

    console.log('send data', data)
    xhr.send(data);
}