修正对FormData的支持

示例代码
```
<!DOCTYPE html>
<html>
<head>
    <meta http-equiv="X-UA-Compatible" content="IE=edge"/>
    <meta charset="utf-8">
    <title>test</title>
    <script src="avalon.js" type="text/javascript"></script>
    <script>
        var avalonAjax;
        var model = avalon.define("test", function (vm) {
            vm.add = function (e) {
                e.preventDefault();
                if (avalonAjax) {
                    var files = this.files;
                    if (files) {
//                        avalon.upload("/upload", document.getElementById("upload"));

//                        avalon.ajax({
//                            form: document.getElementById("upload"),
//                            contentType: "",
//                            url: "/upload",
//                            datatype: "text",
//                            type: "post"
//                        });

                        var formData = new FormData();
                        for (var i = 0, file; file = files[i]; i++) {
                            formData.append(file.name, file);
                        }

                        avalon.ajax({
                            form: formData,
                            contentType: "",
                            url: "/upload",
                            datatype: "text",
                            type: "post"
                        });

//                        var xhr = new XMLHttpRequest();
//                        xhr.onload = function () {
//                            console.log("上传成功！");
//                        };
//                        xhr.open("POST", "/upload", true);
//                        xhr.send(formData);

                    }
                }

            };
        });

        require(["mmRequest"], function (avalon) {
            avalonAjax = avalon.ajax;
            avalon.log(avalonAjax)
        })

    </script>
</head>
<body>
<form action="/upload" id="upload" enctype="multipart/form-data" ms-controller="test"
      ms-on-submit="ajax">
    <input name="email" type=file multiple="multiple" ms-on-change='add()'>
</form>

</body>
</html>
```