melib/imap: FETCH comma-sep list on untagged Recent response

FETCHing RECENT messages when receiving an untagged RECENT response from
the server didn't separate the message numbers with comma but with
space, which is invalid.

melib/src/backends/imap/untagged.rs

@@ -241,12 +241,18 @@ impl ImapConnection {
                         debug!("UID SEARCH RECENT returned no results");
                     }
                     Ok(v) => {
+                        let command = {
+                            let mut iter = v.split(u8::is_ascii_whitespace);
+                            let first = iter.next().unwrap_or(v);
+                            let mut accum = format!("{}", to_str!(first).trim());
+                            for ms in iter {
+                                accum = format!("{},{}", accum, to_str!(ms).trim());
+                            }
+                            format!("UID FETCH {} (FLAGS RFC822)", accum)
+                        };
                         try_fail!(
                             mailbox_hash,
-                            self.send_command(
-                                &[b"UID FETCH", v, b"(FLAGS RFC822)"]
-                                .join(&b' '),
-                                ).await
+                            self.send_command(command.as_bytes()).await
                             self.read_response(&mut response, RequiredResponses::FETCH_REQUIRED).await
                         );
                         debug!(&response);