Fixes golang/go#13126

- GET/HEAD/POST method should not allowed for directory
- Ignore EOFs while detecting the file.

Change-Id: I5ce345408a5ea4ec0dc993631122e6c79fc64398
Reviewed-on: https://go-review.googlesource.com/16799Reviewed-by: Robert Stepanek <robert.stepanek@gmail.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>