BUG/MEDIUM: lua: fix invalid return types in hlua_http_msg_get_body

BUG/MEDIUM: lua: fix invalid return types in hlua_http_msg_get_body

hlua_http_msg_get_body must return either a Lua string or nil. For some
HTTPMessage objects, HTX_BLK_EOT blocks are also present in the HTX buffer
along with HTX_BLK_DATA blocks. In such cases, _hlua_http_msg_dup will start
copying data into a luaL_Buffer until it encounters an HTX_BLK_EOT. But then
instead of pushing neither the luaL_Buffer nor `nil` to the Lua stack, the
function will return immediately. The end result will be that the caller of
the HTTPMessage.body() method from a Lua filter will see whatever object was
on top of the stack as return value. It may be either a userdata object if
HTTPMessage.body() was called with only two arguments, or the third argument
itself if called with three arguments. Hence HTTPMessage.body() would return
either nil, or HTTPMessage body as Lua string, or a userdata objects, or
number.

This fix ensure that HTTPMessage.body() will always return either a string
or nil.

Reviewed-by: Christopher Faulet <cfaulet@haproxy.com>