Use proxy setting from SoapClient when loading XSDs

For the SoapClient, a proxy can be set using $config['soapClientOptions']['proxy_host'] and $config['soapClientOptions']['proxy_port'].
This patch uses those settings to configure the proxy in the libxml stream context for fetching the WSDL.

Resolves  #176 for the 3.x branch.


Very valid concerns. I agree. How about warning/failing if the user tries to set the `proxy` options through the `soapClientOptions`?

The proxy options also include `proxy_login` and `proxy_password`. I guess we should [set these on the stream](http://php.net/manual/en/function.stream-context-create.php#111032) as well?