MINOR: hlua/filters: Add HTTPMessage.closed() to check if input is closed

MINOR: hlua/filters: Add HTTPMessage.closed() to check if input is closed

There is no way for a Lua filter to know if the input of the HTTP message
it is filtering is closed, i.e. if no more data will be received on the
corresponding channel because the end of the stream was reached or an
error was detected.

HTTPMessage.eom() is not suitable to detect that. It relies on the message
framing and is only true when the end of the message was properly
reported. For instance, when a truncated request is received, no end of
message is reported but the input is nonetheless closed.

HTTPMessage.may_recv() is not suitable either because, as its name
implies, it only tells if more data may still be received at this
point. An unfinished message whose input is closed returns false.

Conversely, HTTPMessage.closed() returns true as soon as the input is
closed, whatever the message framing state. However, as for
HTTPMessage.eom(), the result is relative to the data visible by the
current filter. It is only true when all data were already exposed to
it. So if an earlier filter holds back some data, the function returns
false until these data are visible.