bugfix: ompipe used invalid default template

This is a regression from an old change (didn't track it down precisely,
but over a year ago). It used the Forwarding template instead of
the file template (so we have a full syslog header). This fix corrects
it back to previous behaviour, but new scripts that used the wrong
format may now need to have the RSYSLOG_ForwardingFormat template
explicitely be applied.
closes: https://github.com/rsyslog/rsyslog/issues/50