Header already written warning with error responses

[gary-archer]

When writing the tutorial I saw this message in logs occasionally, during tests:

header already sent while reading response header from upstream

This occurs when routing to another location within NGINX during tests.
It seems the below add_header call should use default_type instead.

location /api {
        oauth_proxy on;
        oauth_proxy_cookie_name_prefix "example";
        oauth_proxy_encryption_key $ENCRYPTION_KEY;
        oauth_proxy_trusted_web_origin "https://www.example.com";
        oauth_proxy_cors_enabled on;

        proxy_pass http://localhost/mock-api;
    }

    location /mock-api {
        add_header "content-type" "application/json";
        return 200 '{"message": "API was called successfully with an access token"';
    }
}

[gary-archer]

The main issue was documented in this mailing list:
https://forum.nginx.org/read.php?29,280514,280521#msg-280521

rc = ngx_http_output_filter(request, &output);
ngx_http_finalize_request(request, rc);
return NGX_DONE;

Closed via these PRs:
#19
#21