Bugfix:When Header Repeat assignment #486

bugreport >> [https://github.com/loopj/android-async-http/issues/486](https://github.com/loopj/android-async-http/issues/486)

Just Test Like the code In GetSample.java

```
@Override
    protected void executeSample(AsyncHttpClient client, String URL, Header[] headers, HttpEntity entity, AsyncHttpResponseHandler responseHandler) {
        Header header = new BasicHeader("Authorization", "basic c2piX29mZmxpbmVtYXA6Om9mZmxpbmVtYXA=");
        client.addHeader("Authorization", "basic c2piX29mZmxpbmVtYXA6Om9mZmxpbmVtYXA=");
        client.get(this, URL, new Header[]{ header }, null, responseHandler);
    }
```