Better publish date for podcast json dump

[bas080]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2019.10.16
• I've searched the bugtracker for similar feature requests including closed ones

Description

I have noticed that the upload_date in the json dump is not equal to the xml pubDate value in the podcast xml file. To me it would make sense to either have these date values be similar or that the json would contain a publish_date property.

Example of "The TED Interview" podcast.

    <item>
      <guid isPermaLink="false">prx_179_fae9e468-2558-4796-b84e-d4f1021db169</guid>
      <title>Dan Gilbert on the surprising science of happiness</title>
      <pubDate>Wed, 09 Oct 2019 04:05:00 -0000</pubDate>

{
  "display_id": "TI_S3_E01_Gilbert_seg1",
  "upload_date": "20191001",
  "protocol": "https",
  "_filename": "Dan Gilbert on the surprising science of happiness-TI_S3_E01_Gilbert_seg1.mp3",
  "extractor": "generic",
  "requested_subtitles": null,
  "playlist_uploader": null,

I think the json date is equal to the date the json has been dumped.