Return number of received bytes in the response

Addressing:
{"result":"OK","received":null}
Failed to send SCAP results to the Foreman server: Sent bytes: 29807,
but foreman received:

This regression has been introduced by myself in commit
a7d28107bf042d22db8a3f8292a183068fe468e1. It seems hard to avoid
regressions like this without heavy testing in weak typed languages.
Thanks God we also have strong typed languages!