WebDav server incorrectly outputs two `Content-Length` headers for HEAD requests

[fredsa]

$ curl 'http://meatloaf.local/' -I -H 'User-Agent: MEATLOAF/'

HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 0
Allow: COPY,DELETE,GET,HEAD,LOCK,MKCOL,MOVE,OPTIONS,PROPFIND,PROPPATCH,PUT,UNLOCK
Connection: Keep-Alive
Content-Length: 84
DAV: 1,2
ETag: 6AF3713B2281B7B44AAF87EC6A703CB965BF5E68
Keep-Alive: timeout=5, max=100
Last-Modified: Fri, 23 May 2025 04:03:41 GMT

Having both of these headers present is invalid per RFC 9110: HTTP Semantics.

Content-Length: 0
Content-Length: 84

The presence of the extra Content-Length field causes parsing exceptions in HTTP clients (example).

[fredsa]

This line in webdav_server.cpp Server::doHead generates the non-zero content length:

resp.setHeader("Content-Length", sb.st_size);

I haven't been able to track down where Content-Length: 0 is being generated.