Causes 415 when used with own upstream

while testing doh with an own upstream server (with [doh-proxy of facebookexperimental](https://github.com/facebookexperimental/doh-proxy.git)) I realized, that this tool does not work with my own upstream server

While other clients (as e.g. firefox) are working with it cloudflared does not work. It fails as follows:
```
$ cloudflared proxy-dns --port 5053 --upstream https://myupstream/dns-query
INFO[0000] Starting metrics server addr="127.0.0.1:48603"
INFO[0000] Adding DNS upstream url="https://myupstream/dns-query"
INFO[0000] Starting DNS over HTTPS proxy server addr="dns://localhost:5053"
ERRO[0005] failed to connect to an HTTPS backend "https://myupstream/dns-query" error="returned status code 415"
```
As client I run `dig example.com @127.0.0.1 -p 5053`