[vibe.d] Proper use of preWriteCallback

[Chris via Digitalmars-d-learn]

Maybe a trivial question: How do I use "preWriteCallback" 
properly? It doesn't work properly for me yet (I might not see 
the forest for the trees at the moment). I am trying to set a 
field in the HTTPServerResponse.

http://vibed.org/api/vibe.http.fileserver/HTTPFileServerSettings.preWriteCallback