Fix: Exceptions raised in Soap/Server can leave XML entity loader disabled.

- The original `libxml_disable_entity_loader()` setting was not restored when an exception was raised in `\Zend\Soap\Server::_setRequest()`.
- This caused funny behaviour for subsequent requests which are processed by the same Apache process (also see https://bugs.php.net/bug.php?id=64938 for example).
- Fixed by moving the `libxml_disable_entity_loader()` "cleanup call" a bit further up.
- And instead of forcing entity loading to "false", the previous setting is saved and restored.
- Added a unittest for the intended behaviour.