[2/6] wsdapi: Add code to create SOAP envelope from header and body elements.

[Vincent Povirk]

>> Do we need the namespace for SequenceID?
>
> The namespace needs to be passed in, but it won't appear in the generated
> message, as per:
>
> https://msdn.microsoft.com/en-us/library/windows/desktop/bb513682(v=vs.85).aspx

Is there something special about SequenceId compared to InstanceId and
MessageNumber?