I'm using the request lib [request](https://github.com/request/request) in my project and I need to inform to the API of another company (I can't modify the server) that a transaction was did.
Anyway, to inform to this API I need to use a custom header named `api_key` and when I try to do this, like that:

```
reqOptions = {
    url: 'https://my_url_here.com',
    body: JSON.stringify(transaction),
    headers: {
        'Content-Type': 'text/xml',
        'charset': 'UTF-8',
        'api_key:': 'my_api_key_here'
    }
};

```

I'm getting the error:

```
error: TypeError [ERR_INVALID_HTTP_TOKEN]: Header name must be a valid HTTP token ["api_key:"]

```

How can I set the custom header?