imap: set cselect_bits to CURL_CSELECT_IN initially

... when continuing a transfer from a FETCH response.

When the size of the file was small enough that the entirety of the
transfer happens in a single go and schannel buffers holds the entire
data. However, it wasn't completely read in Curl_pp_readresp since a
line break was found before that could happen. So, by the time we are in
imap_state_fetch_resp - there's data in buffers that needs to be read
via Curl_read but nothing to read from the socket. After we setup a
transfer (Curl_setup_transfer), curl just waits on the socket state to
change - which doesn't happen since no new data ever comes.

Closes #5961

lib/imap.c

@@ -1176,6 +1176,9 @@ static CURLcode imap_state_fetch_resp(struct connectdata *conn, int imapcode,
     else {
       /* IMAP download */
       data->req.maxdownload = size;
+      /* force a recv/send check of this connection, as the data might've been
+       read off the socket already */
+      data->conn->cselect_bits = CURL_CSELECT_IN;
       Curl_setup_transfer(data, FIRSTSOCKET, size, FALSE, -1);
     }
   }