fix: send headers from .setHeaders to server

[vecerek]

Setting the request headers via the setHeaders method is broken. setHeaders only accepted the type Headers which is not a pure object, so destructuring it would not append the headers when calling fetch. One option would be to restrict the type of headers to Record<string, string>. However, that doesn't seem like the direction this package is taking, so the only other option is to handle all possible cases, i.e. Headers, string[][], Record<string, string> and undefined.

The PR also fixes the expected type of input to the setHeaders method, so it matches the type of RequestInit["headers"].

Unfortunately, I wasn't able to meaningfully test the added transformation logic via unit tests. I'm open to suggestions here 🙂

closes #203

[jasonkuhrt]

Thanks @vecerek !