proxy setting for soap client is used for loading WSDL

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.


Since this is related to`DOMDocument::load()` I would prefer that this is pushed in the parent class constructor where the loading is going on. This means that the `$config` property will have to be pushed up to the parent class as well and passed to the parent constructor. That is fine with me.

Also please add an inline comment explaining why this is necessary.