Add feed.updated_deterministic setting

Two main uses cases:

1. Reduce deployment churn in repositories that hold a docsite in
   addition to source code. These currently regenerate and find
   something new commit on every change, even when nothing in the
   site has been changed. The only file changing each time is
   feed.xml.
   For example: https://github.com/qunitjs/qunit/commits/gh-pages

2. Improve reproducibility of the build, as highlighted via
   jekyll/jekyll#7187, by offering a choice
   that simply eliminates use of current time entirely, not even
   having to mock it.

README.md

@@ -45,6 +45,17 @@ feed:
 
 To note, you shouldn't have to do this unless you already have a feed you're using, and you can't or wish not to redirect existing subscribers.
 
+### Deterministic timestamp
+
+By default, the feed will set `<updated>` to the current time (as provided by Jekyll via `site.time`).
+
+Enable the `updated_deterministic` setting to set this instead to a deterministic value based on when the last post was published or updated.
+
+```yml
+feed:
+  updated_deterministic: true
+```
+
 ### Optional front matter
 
 The plugin will use the following post metadata, automatically generated by Jekyll, which you can override via a post's YAML front matter:

lib/jekyll-feed/feed.xml

@@ -6,7 +6,6 @@
   <generator uri="https://jekyllrb.com/" version="{{ jekyll.version }}">Jekyll</generator>
   <link href="{{ page.url | absolute_url }}" rel="self" type="application/atom+xml" />
   <link href="{{ '/' | absolute_url }}" rel="alternate" type="text/html" {% if site.lang %}hreflang="{{ site.lang }}" {% endif %}/>
-  <updated>{{ site.time | date_to_xmlschema }}</updated>
   <id>{{ page.url | absolute_url | xml_escape }}</id>
 
   {% assign title = site.title | default: site.name %}
@@ -51,6 +50,13 @@
     {% assign posts = posts | where_exp: "post", "post.draft != true" %}
   {% endunless %}
   {% assign posts = posts | sort: "date" | reverse %}
+  {% if site.feed.updated_deterministic %}
+    {% assign last_post = posts | first %}
+    {% assign last_updated = last_post.last_modified_at | default: last_post.date | default: site.time %}
+  {% else %}
+    {% assign last_updated = site.time %}
+  {% endif %}
+  <updated>{{ last_updated | date_to_xmlschema }}</updated>
   {% assign posts_limit = site.feed.posts_limit | default: 10 %}
   {% for post in posts limit: posts_limit %}
     <entry{% if post.lang %}{{" "}}xml:lang="{{ post.lang }}"{% endif %}>

spec/jekyll-feed_spec.rb

@@ -116,6 +116,7 @@
 
   context "parsing" do
     let(:feed) { RSS::Parser.parse(contents) }
+    let(:overrides) { { "time" => Time.parse("2018-12-31") } }
 
     it "outputs an RSS feed" do
       expect(feed.feed_type).to eql("atom")
@@ -134,6 +135,24 @@
       expect(feed.generator.version).to eql(Jekyll::VERSION)
     end
 
+    context "with site.feed.updated_deterministic set" do
+      let(:overrides) do
+        {
+          "feed" => {
+            "updated_deterministic" => true,
+          },
+        }
+      end
+
+      it "outputs the last post time" do
+        expect(feed.updated.content).to eql(Time.parse("2016-04-25"))
+      end
+    end
+
+    it "outputs the current time" do
+      expect(feed.updated.content).to eql(Time.parse("2018-12-31"))
+    end
+
     it "includes the items" do
       expect(feed.items.count).to eql(10)
     end