Fix empty SOAPAction header

[davidolrik]

SOAPAction header should never be "None".

When self.operation.soapaction is None it was stringifyed into "None" when it should be "".

This fixed the following error when calling a Soap 1.1 service on Camunda:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <soap:Fault>
      <faultcode>soap:Server</faultcode>
      <faultstring>The given SOAPAction None does not match an operation.</faultstring>
    </soap:Fault>
  </soap:Body>
</soap:Envelope>

[jdury]

+1 same issue, and fix works like a charm