Fixed behavior that not updates soapenv body from custom MessagePlugin

[amammi]

@cackharot I've encountered the problem of updating soap request message with a custom MessagePlugin like this (it's just an example):

class HelloSOAPWorldPlugin(MessagePlugin):

    def sending(self, context: MessageContext):

        context.envelope = b'Hello World SOAP!'

the result is that request is the same and is not updated like in the custom MessagePlugin.

I've found this fix and, since I'm not familiar with this project, feel free to update this PR.

Thanks in advance.