Adding custom envelope key option for server

[gamzeoguz]

No description provided.

[w666]

I rebased this branch so it has the latest changes and to run tests.

[w666]

Also, seems like error responses also should contain custom key

<?xml version="1.0" encoding="utf-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
  xmlns:tns="http://example.com/stockquote.wsdl" xmlns:xsd1="http://example.com/stockquote.xsd">
  <soapenv:Body>
    <soap:Fault>
      <soap:Code>
        <soap:Value>SOAP-ENV:Server</soap:Value>
        <soap:Subcode>
          <soap:Value>InternalServerError</soap:Value>
        </soap:Subcode>
      </soap:Code>
      <soap:Reason>
        <soap:Text>Error: Failed to parse the SOAP Message body</soap:Text>
      </soap:Reason>
    </soap:Fault>
  </soapenv:Body>
</soapenv:Envelope>

[w666]

Pushed changes to tests, but they still do not work as request is not valid.