请求 http.GET 时发送的 Angular2 OPTIONS 方法

我正在try 向angular2应用程序添加basic authentification.

public login() {
    // Set basic auth headers
    this.defaultHeaders.set('Authorization', 'Basic ' + btoa(this.username + ':' + this.password));

    console.log('username', this.username)
    console.log('password', this.password)
    console.log(this.defaultHeaders)

    // rest is copy paste from monbanquetapiservice
    const path = this.basePath + '/api/v1/development/order';        

    let req = this.http.get(path, { headers: this.defaultHeaders });
    req.subscribe(
        _ => { },
        err => this.onError(err)
    );
}

我希望看到的是一个GET请求，带有我输入的Authorization个标题.

但我看到的是第一个OPTIONS，标题如下:

OPTIONS /api/v1/development/order HTTP/1.1
Host: localhost:8080
Connection: keep-alive
Access-Control-Request-Method: GET
Origin: http://localhost:3000
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.110 Safari/537.36
Access-Control-Request-Headers: authorization, content-type
Accept: */*
Referer: http://localhost:3000/
Accept-Encoding: gzip, deflate, sdch
Accept-Language: en-GB,en-US;q=0.8,en;q=0.6,fr;q=0.4

因为我的服务器不允许在这个url上使用OPTIONS，所以我收到一个错误.

我知道有些方法，比如PUT或POST，会先发送一个OPTIONS方法来预处理请求，但GET没有.

为什么angular2的http首先发送选项？

谢谢