optional curl_setopt settting i.e. for proxy support

Since a6378f224f9e9fe93379b38fbac306423727c397 there is curl support. 

Can you please make optional curl_setopt settings available when using curl i.e. to use a proxy. 

```
curl_setopt ($ch, CURLOPT_PROXY, $proxy);
curl_setopt ($ch, CURLOPT_PROXYUSERPWD, $proxyuser);
```

Since _requestWithCurl() is private I don't see how to do that without changing the core.
