一、formData.get('key')获取表单对象属性的值
var formData = new FormData(form);
console.log(formData.get('username'));
二、formData.set('key','value')设置表单对象属性的值（设置覆盖已有键名）
var formData = new FormData(form);
console.log(formData.set('username','andy'));
三、formData.delete('key')删除表单对象属性的值
var formData = new FormData(form);
console.log(formData.delete('username'));
四、formData.append('key')向表单对象追加属性值（追加保留原有，再追加）
var formData = new FormData(form);
console.log(formData.append('age',20));
