Return https if r.TLS is not nil (#126)

`go-httpbin` was incorrectly returning `http` as the scheme for its URL
if you ran it with TLS specified via `HTTPS_CERT_FILE` and
`HTTPS_KEY_FILE`. Update it to return https as the scheme in this case
by checking if `r.TLS` is not nil

Before:

```
$ mkcert test
$ docker run -e HTTPS_CERT_FILE='/tmp/test.pem' -e HTTPS_KEY_FILE='/tmp/test-key.pem' -p 8080:8080 -v $(pwd):/tmp mccutchen/go-httpbin
$ curl -sk https://localhost:8080/get | jq -r .url
http://localhost:8080/get
```

After (TLS):

```
$ docker run -e HTTPS_CERT_FILE='/tmp/test.pem' -e HTTPS_KEY_FILE='/tmp/test-key.pem' -p 8080:8080 -v $(pwd):/tmp llimllib/go-httpbin
$ curl -sk https://localhost:8080/get | jq -r .url
https://localhost:8080/get
```

After (no TLS):

```
$ docker run -p 8080:8080 llimllib/go-httpbin
$ curl -sk http://localhost:8080/get | jq -r .url
http://localhost:8080/get
```

I got the idea for how to check the scheme from [this SO
post](https://stackoverflow.com/a/61446922/42559)