fix: actually use "body" property

[wcelery]

I've noticed, that when you use method.type: 'body' nothing really happens and we're not sending payload. This PR fixes it.

[niels-bosman]

@wcelery Thanks for contributing! In the case that the body is not set, doesn't this send 'null' or 'undefined' as the body? I would like to have a check here to make sure we don’t send stringified null values to the API.

[wcelery]

@niels-bosman Hi. If body is undefined, no payload will be sent (JSON.stringify handles it for us) and if body is null, we're converting null -> undefined by JSON.stringify's replacer function.

[kgoedecke]

@wcelery @niels-bosman thanks a lot for fixing this, looks good to me!