Comment 6 for bug 2039251

I couldn't really figure out where it actually sets the Content-Length, it doesn't seem to happen in lazr.restful itself, but maybe it's fixed by returning `None` instead of `b""` in places where it says "# The conditional GET succeeded. Serve nothing."?

I'm also pretty sure it's caused by sending a Content-Length 0 when a 304 is generated, basically telling Squid that the new content for the cached file should be of length 0, emptying the cached object.