Secret parameters aren't sealed when dumped via RPC

[mururu]

• fluentd 0.12, 0.14

$ bundle exec bin/fluentd -c worker.conf
2017-05-19 13:37:24 +0900 [info]: reading config file path="worker.conf"
2017-05-19 13:37:24 +0900 [info]: starting fluentd-0.12.35
2017-05-19 13:37:24 +0900 [info]: gem 'fluentd' version '0.12.35'
2017-05-19 13:37:24 +0900 [info]: gem 'fluent-plugin-forest' version '0.3.3'
2017-05-19 13:37:24 +0900 [info]: adding match pattern="**" type="null"
2017-05-19 13:37:24 +0900 [info]: adding source type="fake"
2017-05-19 13:37:24 +0900 [info]: using configuration file: <ROOT>
  <system>
    rpc_endpoint 127.0.0.1:9000
  </system>
  <source>
    @type fake
    secret xxxxxx
  </source>
  <match>
    @type null
  </match>
</ROOT>

$ curl http://127.0.0.1:9000/api/config.dump
{"ok":true}%

2017-05-19 13:37:30 +0900 [info]: dump in-memory config
2017-05-19 13:37:30 +0900 [info]: <ROOT>
  <system>
    rpc_endpoint 127.0.0.1:9000
  </system>
  <source>
    @type fake
    secret very-secret-string0
  </source>
  <match>
    @type null
  </match>
</ROOT>