DefaultServlet with ETag

In my opinion, the default servlet should make use of ETags by default or at least be able to be easily configured to use Etags for serving static contents from a war.

io.undertow.servlet.handlers.DefaultServlet.serveFileBlocking looks quite prepared for handling ETags but io.undertow.server.handlers.resource.PathResource.eTag is always null because io.undertow.server.handlers.resource.PathResourceManager.eTagFunction is always NULL_ETAG_FUNCTION because also io.undertow.server.handlers.resource.PathResourceManager.Builder.eTagFunction is always NULL_ETAG_FUNCTION.

I might have missed something. Certainly, the documentation for that feature is not so easy to find if it exists at all. I have not found anything.

At first glance it looks more like it would have worked in earlier versions and some portion of code has been lost somewhere and now eTagFunction can not be changed any longer and stays NULL_ETAG_FUNCTION which is of no use.

I have filled in rather too few field of this ticket than too many. I'm not totally sure if servlet is the correct component and also have not checked other affected versions than 2.2.2.Final which certainly is.