Fetch module typing and use of the Headers class

[OlliV]

This might be a good idea:
https://github.com/zeit/fetch/pull/43/files

export default function SetupFetch(
	fetchModule?: FetchModule,
	agentOptions?: http.AgentOptions | https.AgentOptions
): Fetch;

Currently we avoid the issue by using the Headers class as implemented by node-fetch which might, in theory, be different and incompatible in some other implementation.

However, let's ensure that we align with Fetch Standard whenever possible.