Have redbean show zip listing as default / handler

If an "index.lua" or "index.html" doesn't exist in zip file or the
filesystem, and no redirects have been defined for it either, then
redbean will render a listing of the zip central directory content
only if the request uri points to the root path.

tool/net/redbean-xhr.lua

@@ -1,2 +1,3 @@
 -- redbean xhr handler demo
+SetHeader('Vary', 'X-Custom-Header')
 SetHeader('X-Custom-Header', 'hello ' .. GetHeader('x-custom-header'))