update SoapClient send method

[OliverWBurke]

The sending plugin currently has no effect on the request that is sent There is a similar change that has been made on the jurko fork to resolve this issue here, https://github.com/Affirm/suds-jurko/blob/master/suds/client.py#L604 That fork has fallen behind, so I feel that it is worth bringing the fix here The documentation around the plugin usage could do with a bit of an update as well, I'd be happy to update that as well if we feel that this is a change that would be brought in