fix #1320 returning Last-Modified header along with ETag

This is a targeted fix addressing vfs and etag/Last-Modified header.   

- Only `/vfs/` endpoint that we will return Last-Modified header along with etag.
- Only GET, PUT and HEAD Http verbs.


Yes, it did clear Content-Type header but not Content-Length (Content-Length: 0).  If OK, I will leave the codes as is to achieve the exact same behavior as not setting response.Content at all.