Ignore headers by default?

There seems to be more surprises by how hashing operates, considering `headers`.

So, either there could be a `hash` option for customizing...

Or, `normalize` can choose to return the normalized object (instead of mutating), where returning `undefined` will keep normal normalization.

(I still think that Polly.js does a good job of hashing headers with `matchRequsestsBy`: https://netflix.github.io/pollyjs/#/configuration?id=defaults)