It was possible get the status code in the HTTP response and the one
received from the server. Thanks to 'txn.status', it is now possible to get
the transaction status code. It is equivalent to '%ST' in log-format.
Most of time, it is the same than 'status', except if the status code of the
HTTP reply does not match the one used to interrupt the transaction. For
instance, an error file use mapped on 400 containing a 404.
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