BUG/MINOR: lua: Lua applets must not use http_txn

BUG/MINOR: lua: Lua applets must not use http_txn

In certain circumstances (eg: Lua HTTP applet called from a
TCP ruleset before http_process_request()), the HTTP TXN is not
yet fully initialized so some information it contains cannot be
relied on. Such information include the HTTP version, the state
of the expect: 100-continue header, the connection header and
the transfer-encoding header.

Here the bug only turns something which already doesn't work
into something wrong, but better avoid any references to the
http_txn from the Lua code to avoid future mistakes.

This patch should be backported into 1.6 for code consistency.