Rewrite: proxy_set_header gets set in other ways, too.

`proxy_set_header` doesn't only get set when `$uses_echo_module` is true. It also gets set when `$self->accept_header` and `$is_duckduckgo` are true. So it's important to check if at least one of them is true because only one of them is enough to override all the `proxy_set_headers` that were set beforehand.