[fetch] Error Handle Chinese character for Charset=UTF-8 (#102)

I'm using Chrome(45.0.2454.15 beta (64-bit)) and Firefox(33.1.1) on Mac OS X 10.10.4. Both is supposed `fetch`.
I got messy code when the server response text has Chinese.

This is steps you can reproduce:
1. open http://xueqiu.com/
2. Open devtool console and type: 

```
var url = 'http://xueqiu.com/imgroups/search.json?q=%E5%A4%A7%E5%8A%9B&count=3&sg=1';
fetch(url, { method: 'get', credentials: 'include' }).then(function (res) {
  return res.json();
}).then(function (json) {
  return console.log('receive: ' + JSON.stringify(json));
});
```
Here is the response headers
![image](https://cloud.githubusercontent.com/assets/948896/9077282/f432b638-3b5e-11e5-8228-677cf10a83ce.png)

![image](https://cloud.githubusercontent.com/assets/948896/9077286/065d822a-3b5f-11e5-9877-7b45b3775c3b.png)


response I got **大力**  which should be **大力**。This should not be an issue of server response, since it works when using `XMLHTTPRequest`.


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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/102