extract_placeholders cannot extract ${hostname} properly

[okkez]

• fluentd v0.14.9
• Debian GNU/Linux

config:

<source>
  @type dummy
  tag raw.dummy
  dummy [
  {"host":"66.102.9.80", "message":"test1"},
  {"host":"66.102.9.80", "message":"test2"},
  {"host":"66.102.9.80", "message":"test3"},
  {"host":"66.102.9.80", "message":"test4"},
  {"host":"66.102.9.80", "message":"test5"}
  ]
</source>

<match raw.**>
  @type geoip
  tag ${tag[0]}.geoip.${hostname}
  geoip_lookup_key  host
  #geoip_database    "/path/to/your/GeoIPCity.dat"
  <record>
    city            ${city["host"]}
    latitude        ${latitude["host"]}
    longitude       ${longitude["host"]}
    country_code3   ${country_code3["host"]}
    country         ${country_code["host"]}
    country_name    ${country_name["host"]}
    dma             ${dma_code["host"]}
    area            ${area_code["host"]}
    region          ${region["host"]}
  </record>
  <buffer tag, hostname>
    flush_interval    1s
  </buffer>
  <inject>
    hostname_key hostname
  </inject>
</match>

<match raw.**>
  @type stdout
</match>

https://github.com/okkez/fluent-plugin-geoip/blob/migrate-to-v0.14-api/lib/fluent/plugin/out_geoip.rb

I'm trying to migrate fluent-plugin-geoip's out_geoip to Fluentd v0.14 API.
I want to extract placeholders ${hostname} using hostname field added by inject plugin helper.

Expected behavior:

tag will include hostname added by inject plugin helper.

Actual behavior:

tag does not include hostname added by inject plugin helper.