This event is signalled by act_dispatch() in inc/actions.php after preparing the headers and before loading the template.
The passed Doku_Event object has the fields: $data and $result.
The $data field is an array of header strings. This array has default one entry:
'Content-Type: text/html; charset=utf-8'.
The BEFORE handler of this event can add/modify/delete the headers in the $data field. The event data must remain an array, even if emptied. The default action act_sendheaders() will output the headers, but this can be prevented by preventDefault().
The $result field is always null, because the default action don't set this field. The AFTER event handlers comes (default) after outputting the headers.