rgw: Swift API. The second way of specifying desirable response format.

rgw: Swift API. The second way of specifying desirable response format.

OpenStack Object Storage API v1 defines two ways for the client to tell which response format it understands.
Until now RGW looked at query variable 'format' only. This commit implements the second way of setting the format by
providing 'Accept' HTTP-header with the desirable response mime-type.

Backport: hammer
Fixes: #10746
Reviewed-by: Yehuda Sadeh <yehuda@redhat.com>
Signed-off-by: Dmytro Iurchenko <diurchenko@mirantis.com>