Setting XHR response type fails due to xhr.responseType being modified before XHR is opened

http-browserify accepts a 'responseType' parameter to set the XHR response type. However, in Firefox 31 at least, the logic to set the response type always fails because it is set before xhr.open() is called.

The call to set 'xhr.responseType = params.responseType' fails with 'An attempt was made to use an object that is not, or is no longer, usable' ala. http://stackoverflow.com/questions/13216903/get-binary-data-with-xmlhttprequest-in-a-firefox-extension

定位 http-browserify 中对 responseType 的处理，并将其赋值操作与 xhr.open() 调用进行比较。如果可能，重现 Firefox 失败的情况，然后验证在打开 XHR 后设置 responseType 可以成功，且不会出现所报告的不可用对象错误。

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