add namespace for soap envelope

Add namespace for soap envelope if namespace is not soap.
for exemple:
```xml
<soapenv:Envelope xmlns:soapenv="http://www.w3.org/2003/05/soap-envelope" xmlns:web="http://webservices.secureholiday.net/">
  <soapenv:Header/>
  <soapenv:Body/>
</soapenv:Envelope>
```

do this:
```php
$soap = new SoapClient($wsdl, $params);
$soap
  ->setNamespaces(
    array(
      'soapenv:Envelope' => array(
        'uri'      => 'http://www.w3.org/2003/05/soap-envelope',
        'prefix'   => 'soapenv',
        'envelope' => true
      ),
      'web:attr'      => 'http://webservices.secureholiday.net/'
    )
  );
```