Avoid stale headers by supplying them as a function

One of the most common usages for headers is authentication. However, if one provides a token as part of a header to this library, its value will remain the same forever, even through retries and whatnot. If at a given moment the token expires and needs to be refreshed, there will be no hope in reconnecting, so the retry mechanism becomes quite pointless.

I propose to allow passing "headers" in as a function, in which case it will be called every time `fetch()` is, in order to recompute the potential dynamic values in them.