content type incorrect on proxy request

On a website traffic is flowing in via http for location /instance we need the traffic to be secure using SSL and https. When redirecting to https the request content-type is "text/xml" when it actually should be "application/json". Do we need to explicitly set something in the proxy header to "application/json". We have tried add_header Content-type "application/json" in the http config and that didn't make a difference. Any suggestions on what I'm doing wrong would be greatly appreciated.

Http configuration

location /instance {
proxy_pass https://instancehost:9443/instance;
proxy_redirect http://localhost.com https://localhost.com;
proxy_set_header X-xmgr-proxy-domain http://localhost.com:80;
proxy_set_header X-xmgr-proxy /instance;
proxy_set_header Access-Control-Allow-Origin "*";
proxy_set_header Access-Control-Allow-Headers "Origin, X-Requested-With, Content-Type, Accept";
proxy_ssl_certificate /data/nginx/certs/abc.crt;
proxy_ssl_certificate_key /data/nginx/certs/abc.key;
proxy_ssl_trusted_certificate /etc/pki/tls/certs/abc-bundle.crt;
proxy_ssl_protocols SSLv2 SSLv3 TLSv1 TLSv1.1 TLSv1.2;
proxy_hide_header Content-Type;
add_header Content-type "application/json"
}

Setting the content-type in the header did not work we are still receiving a 204 error.

https configuration

location /instance {
proxy_pass https://instancehost.com:9443/instance;
proxy_set_header X-xmgr-proxy-domain https://localhost.com:443;
proxy_set_header X-xmgr-proxy /instance;
proxy_set_header Access-Control-Allow-Origin "*";
proxy_set_header Access-Control-Allow-Headers "Origin, X-Requested-With, Content-Type, Accept";
proxy_ssl_certificate /data/nginx/certs/abc.crt;
proxy_ssl_certificate_key /data/nginx/certs/abc.key;

}