rgw: http_client: propagate send_data_hint differently

rgw: http_client: propagate send_data_hint differently

Fixes: http://tracker.ceph.com/issues/36092
send_data_hint is passed so that the lower http client layer
to know in in cases where it's an http request that usually doesn't
send data (e.g., GET) that the request will actually send data,
and initialize the curl client correctly. In a case that was
used by the elasticsearch mechanism it wasn't passed correctly.
Now made it a data member in RGWHTTPClient, which also cleans
the interface a bit.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>