Add pubdate for repository rss and add some tests (#33411) (#33416)

Backport #33411 by @lunny

Fix #33291

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/web/feed/branch.go

@@ -43,6 +43,7 @@ func ShowBranchFeed(ctx *context.Context, repo *repo.Repository, formatType stri
 			},
 			Description: commit.Message(),
 			Content:     commit.Message(),
+			Created:     commit.Committer.When,
 		})
 	}
 

routers/web/feed/file.go

@@ -55,6 +55,7 @@ func ShowFileFeed(ctx *context.Context, repo *repo.Repository, formatType string
 			},
 			Description: commit.Message(),
 			Content:     commit.Message(),
+			Created:     commit.Committer.When,
 		})
 	}