radosgw: receiving unexpected error code while accessing an non-existing object by authorized not-owner user

This patch fixes a bug in radosgw swift compatibility code,
that is, if a not-owner but authorized user access a non-existing
object in a container, he wiil receive unexpected error code,
to repeat this bug, do the following steps,

1 User1 creates a container, and grants the read/write permission to user2

curl -X PUT -i -k -H "X-Auth-Token: $user1_token" $url/$container
curl -X POST -i -k -H "X-Auth-Token: $user1_token" -H "X-Container-Read:
$user2" -H "X-Container-Write: $user2" $url/$container

2 User2 queries the object 'obj' in the newly created container
by using HEAD instruction, note the container currently is empty

curl -X HEAD -i -k -H "X-Auth-Token: $user2_token" $url/$container/obj

3 The response received by user2 is '401 Authorization Required',
rather than the expected '404 Not Found', the details are as follows,

HTTP/1.1 401 Authorization Required
Date: Tue, 16 Apr 2013 01:52:49 GMT
Server: Apache/2.2.22 (Ubuntu)
Accept-Ranges: bytes
Content-Length: 12
Vary: Accept-Encoding

Signed-off-by: Yunchuan Wen <yunchuanwen@ubuntukylin.com>
Signed-off-by: Li Wang <liwang@ubuntukylin.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>