Set comment created_at/updated_at types to support null.

The `review` field of `PullRequestReviewEvent` isn't quite the same as a
`Comment`, even though we are pretending they are the same. The
`submitted_at` is optional, which was causing deserialization errors.

I was inclined to stop treating pull request review comments the same as
comments, since their fields don't quite line up. However, that is a
much larger change, and generally makes handling pr review comments more
awkward.

src/github.rs

@@ -363,9 +363,9 @@ pub struct Comment {
     pub body: String,
     pub html_url: String,
     pub user: User,
-    #[serde(alias = "submitted_at")] // for pull request reviews
+    #[serde(default, alias = "submitted_at")] // for pull request reviews
     pub created_at: chrono::DateTime<Utc>,
-    #[serde(alias = "submitted_at")] // for pull request reviews
+    #[serde(default, alias = "submitted_at")] // for pull request reviews
     pub updated_at: chrono::DateTime<Utc>,
     #[serde(default, rename = "state")]
     pub pr_review_state: Option<PullRequestReviewState>,

src/lib.rs

@@ -185,7 +185,7 @@ pub async fn webhook(
             payload.pull_request.pull_request = Some(PullRequestDetails::new());
 
             // Treat pull request review comments exactly like pull request
-            // review comments.
+            // comments.
             github::Event::IssueComment(github::IssueCommentEvent {
                 action: match payload.action {
                     github::PullRequestReviewAction::Submitted => {