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});
}