A simple dynamic object that can be used to build soap calls without
implementing KvmSerializable
Essentially, this is what goes inside the body of a soap envelope - it is the
direct subelement of the body and all further subelements
Instead of this this class, custom classes can be used if they implement the
KvmSerializable interface.
Write <?xml declaration with encoding (if encoding not null)
and standalone flag (if standalone not null)
This method can only be called just after setOutput.