mod_pubsub_summary: Use pre-escaped UTF-8 sequence for compat This silences a warning about using the non-existent global 'utf8' each time this line was hit. The utf8 library was introduced in Lua 5.3, so the warning would appear with 5.1 and 5.2.

This module produces more detailed HTTP logs for Prosodys built-in HTTP
server. The format is similar to that of Apache and go into Prosodys
normal logs at the `info` level.