[Zend\Soap\Server][NEW] add a getSoap method, return the internal instance

Currently, we are no able to access of the internal SoapServer instance. But in certain case (especially when we need to send a soapfault with the setReturnResponse at true) we need an access to the internal instance.
So this pull request adds a proxy method getSoap() which returns the internal SoapServer instance.

Code is unit-tested