rgw: send http status reason explicitly in fastcgi

There are issues in certain versions of apache 2.4, where the reason is
not sent back. Instead, just provide the reason explicitly.

Backport: firefly, giant

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
Merge pull request #2789 from ceph/wip-rgw-explicit-reason

rgw: send http status reason explicitly in fastcgi