mod_pastebin: Prevent header table form being mutated default_headers was mutated, probably by mod_http_errors and ended up with multiple content-type entries, which led to issues in browsers which would sometimes treat the paste as html

This module writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.