Allow call with SOAP envelope

[yousuf-particle]

soap.call method takes request argument and embeds the request in SOAPEnvelope.

/ /SOAP envelope capable of namespace prefixes
		soapEnvelope := SOAPEnvelope{
			XmlNS: XmlNsSoapEnv,
		}
		soapEnvelope.Headers = s.headers
		soapEnvelope.Body.Content = request
		envelope = soapEnvelope

This generates the message of the form :

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Header>
    <Headers>
        ............
 </Headers>
  </soap:Header>
  <soap:Body>
    <Content xmlns="">
     ......
    </Content>
  </soap:Body>
</soap:Envelope>

This PR allows the option to override this by passing in an envelope. Which would allow for removing and tags.