Test if X-Sendfile header is working
According to the source on github X-Sendfile headers will be stripped.
If I'm skimming the file correctly, it's only logging success if it's been compiled in debug mode.
You could check memory usage of sending large files with and without xsendfile.
They are being stripped, simply because having them present will prevent one of the reasons to use it, namely having the file served without the recepient knowing the location of the file being served.