net/http: avoid sending unspecified time for directories

Change applies to sendFile.
This is already done for sendContent.

Change-Id: If43d9ab99e6e66a1363b08e0bdcceb57df1f855c
GitHub-Last-Rev: 1c47620a09a6f5e2b3d777fadaad6e0189de4af5
GitHub-Pull-Request: golang/go#34631
Reviewed-on: https://go-review.googlesource.com/c/go/+/198139Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/fs.go

@@ -610,7 +610,7 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 			writeNotModified(w)
 			return
 		}
-		w.Header().Set("Last-Modified", d.ModTime().UTC().Format(TimeFormat))
+		setLastModified(w, d.ModTime())
 		dirList(w, r, f)
 		return
 	}