HEAD requests to FileResource have Content-Length overwritten

When using the ResourceHandler with FileResourceManager and asking for the size of a file with a HEAD request Undertow always respond with Content-Length: 0.

Sample code and discussion in the referenced forum post.

I think the problem is not only in the ResourceHandler, because if I set the Content-Length header in it, it still comes out as 0. I suspect the core code has no notion of empty entity responses in the HEAD case.