Readme improvement

In this example in first line we extends BaseClient with http namespace,
but later we use StreamedResponse and BaseRequest without namespace.
So I think it's better to add http prefix to them too.
```
class UserAgentClient extends http.BaseClient {
final String userAgent;
final http.Client _inner;

UserAgentClient(this.userAgent, this._inner);

Future send(BaseRequest request) {
request.headers['user-agent'] = userAgent;
return _inner.send(request);
}
}
```

Find the README example containing the UserAgentClient code shown in the issue and compare its namespace usage. Update StreamedResponse and BaseRequest to use the http prefix, then verify the example remains valid Dart code.

Written by the indexing model from the issue text.