close response body when doing _cbauth requests

Otherwise, apparently connection is not reused.

Change-Id: I7fc93c6aeec73e633f1a582f9cf15378cf016920
Reviewed-on: http://review.couchbase.org/44496
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>