Ignore SL_EAGAIN

It was observed that sl_Send can return SL_EAGAIN, despite the fact that
we only call write_to_socket if it was reported as writeable.

PUBLISHED_FROM=e4cbacb3f16bb7abb479222d7dcbfbe113a39161

mongoose.c

@@ -10626,7 +10626,7 @@ void mg_set_non_blocking_mode(sock_t sock) {
 }
 
 static int mg_is_error(int n) {
-  return (n < 0 && n != SL_EALREADY);
+  return (n < 0 && n != SL_EALREADY && n != SL_EAGAIN);
 }
 
 void mg_if_connect_tcp(struct mg_connection *nc,