rpc: fix crash in verbose mode
    
    Fix a crash caused by feeding incorrect data to `json.dumps()` in
    `JSONServerProxy.__request()` introduced by commit
    8159c2883bf66980582d1227c364df4e592bdd7e.
    
    https://pagure.io/freeipa/issue/6734
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>