Fix processing
    error_page 502 504 /zero;
    location = /zero { return 204; }

The bug has been introduced in r3634.
The fix also allow to use:
    error_page 502 504 = /zero;
    location = /zero { return 200; }

This case still does not work:
    error_page 502 504 /zero;
    location = /zero { return 200; }