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

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summary: Log a message once per minute
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This module sends `-- MARK --` to the log once per minute. This may be
useful to give a sense of how busy the server is or see that logging and
timers are still working.