guid value in RSS feed plugin

[frenchvandal]

I opened the PR #831 to add Atom feed support and implement some polishing touches on the RSS output of the feed plugin. I originally wanted to correct the guid entry too but I am not sure about this.
Currently the plugin builds this output:

<guid isPermaLink="false">https://example.com/page5/</guid>

with:

guid: {
  "@isPermaLink": false,
  "#text": item.url,
},

The spec says that isPermaLink is optional and its default value is true. This false with an URL seems a bit contradictory to me.

Should the plugin instead set this value to true:

<guid isPermaLink="true">https://example.com/page5/</guid>

or, better, no value at all as it would be redundant with the spec behaviour:

<guid>https://example.com/page5/</guid>

I am no RSS expert so I prefer to check before adding this change to PR #831.

[oscarotero]

I didn't make the initial version of the plugin but it's from a PR 3 years ago and the guid value was already there. Looking at the commit with the message "Debug rss plugin", my guess is that it was introduced for compatibility with some RSS readers or validators. Since the RSS item has the link value with the URL, I'm not sure about the utility of <guid isPermalink="true">, because it does the same. I guess the purpose of guid is to uniquely identify the item and avoid duplicated entries. In the context of the Lume plugin, that uses the URL for both link and guid, it's a bit redundant. But the isPermalink=false attribute allows (in the future) adding a configuration option to use a different value guid that is not a URL.