luci-base: fix luci.http.close()

luci-base: fix luci.http.close()

Ensure that `http.write()` or template rendering operations after a call
to `http.close()` do not produce additional output. This is required for
certain legacy Lua apps which invoke write and close operations in the
middle of a server side cbi rendering process.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>