fake db infos when dropbox=true and the user isn't authorized.

All sizes and counts informations are set to 0 so an unauthorized user
view the db as empty.

    $ curl -XGET http://localhost:5984/testdb
    {"db_name":"testdb","doc_count":0,"doc_del_count":0,"update_seq":0,"purge_seq":0,"compact_running":false,"disk_size":0,"data_size":0,"instance_start_time":"1321948511448569","disk_format_version":6,"committed_update_seq":0}

    $ curl -XGET http://admin:test@localhost:5984/testdb
    {"db_name":"testdb","doc_count":1,"doc_del_count":0,"update_seq":2,"purge_seq":0,"compact_running":false,"disk_size":8283,"data_size":279,"instance_start_time":"1321948511448569","disk_format_version":6,"committed_update_seq":2}