Custom hostnames - addHostname does not work

When I try to execute the following:

```
new CustomHostnames($this->adapter)->addHostname(
"zone_id",
"mydomain.com",
);
```

I get the error message: `Unable to decode the JSON request body. Please check your input and try again.`.

I tried to modify the `addHostname`-function and remove the ssl-array, and by doing that, I could successfully create a custom hostname.

I am using a free-plan account on cloudflare.