Differentiate between publish and update time

The `pubDate` field of an item in the feed is meant to be time the item
was published. It should not be bumped if the item is updated in the
backend.

* Introduce a new `published_at` field on `Item`
* Update the Mixcloud and YouTube backends to fill this field
* Use the `published_at` field on `Item` for the `<pubData>` item
  subelement

src/backends.rs

@@ -109,6 +109,9 @@ pub(crate) struct Item {
     /// The URL of the image of the item.
     pub(crate) image: Option<Url>,
 
+    /// The timestamp the item was published.
+    pub(crate) published_at: DateTime<Utc>,
+
     /// The timestamp the item was last updated.
     pub(crate) updated_at: DateTime<Utc>,
 }

src/backends/mixcloud.rs

@@ -157,7 +157,10 @@ pub(crate) struct Cloudcast {
     /// The tags of the cloudcast.
     pub(crate) tags: Vec<Tag>,
 
-    /// The time the feed was created/started.
+    /// The time the feed was created.
+    pub(crate) created_time: DateTime<Utc>,
+
+    /// The time the feed was updated.
     pub(crate) updated_time: DateTime<Utc>,
 
     /// The original URL of the cloudcast.
@@ -227,6 +230,7 @@ impl From<Cloudcast> for Item {
             guid: cloudcast.slug,
             keywords,
             image: Some(cloudcast.pictures.large),
+            published_at: cloudcast.created_time,
             updated_at: cloudcast.updated_time,
         }
     }

src/backends/youtube.rs

@@ -224,7 +224,9 @@ impl From<YouTubeVideoWithStream> for Item {
             .date()
             .and_hms_opt(12, 0, 0)
             .expect("Invalid hour, minute and/or second");
-        let updated_at = DateTime::from_utc(timestamp, Utc);
+        let published_at = DateTime::from_utc(timestamp, Utc);
+        // There is no updated at timestamp available, really.
+        let updated_at = published_at;
 
         Item {
             title: video.title().to_string(),
@@ -236,6 +238,7 @@ impl From<YouTubeVideoWithStream> for Item {
             guid: id,
             keywords,
             image,
+            published_at,
             updated_at,
         }
     }

src/feed.rs

@@ -125,7 +125,7 @@ fn construct_item(
         .categories(categories)
         .enclosure(Some(enclosure))
         .guid(Some(guid))
-        .pub_date(Some(item.updated_at.to_rfc2822()))
+        .pub_date(Some(item.published_at.to_rfc2822()))
         .itunes_ext(Some(itunes_ext))
         .build()
 }