When If-Range does not match and the requested resource is
available, server should return a "200 OK" response to client.
Currently server returns "200 OK" when the request method is
GET, but "206 Partial Content" when method is HEAD.
This change fixed this inconsistency.

Change-Id: I5ad979919f4f089baba54a4445b70ca38471a906
Reviewed-on: https://go-review.googlesource.com/54110
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>