rgw: correct "If-Modified-Since" handle.

Normally client will use `Last-Modified` of server returned, as the
value of `If-Modified-Since`. So the `ctime` is exactly equal to
`*mod_ptr`, and rgw should return `Not Modified`, otherwise the cache
will never worked.

Signed-off-by: VRan Liu <gliuwr@gmail.com>
Merge pull request #2725 from CzBiX/rgw-modified-since

rgw: correct "If-Modified-Since" handle.

Reviewed-by: Yehuda Sadeh <yehuda@redhat.com>