Zorba I have try and I haven't problem:
//proxy page...
        esri.config.defaults.io.proxyUrl = "ProxyPage/proxy.ashx";
        function requestSucceeded(response, io) {
            console.log("Succeeded: ", response);
        }
        function requestFailed(error, io) {
            console.log("Failed: ", error);
        }
            //function to edit features
            function addFeature(){
                var url = esri.urlToObject("http://sampleserver3.arcgisonline.com/ArcGIS/rest/services/SanFrancisco/311Incidents/FeatureServer/1/addFeatures?features=[{'attributes' : {'agree_with_incident' : '2', 'notes' : 'problemo grande1'}}]&f=json");
                var requestHandle = esri.request({
                url:url.path,
                content:url.query,
                callbackParamName:"callback",
                load:requestSucceeded,
                error:requestFailed
            }, {useProxy:true, usePost:true});
            }