fix: resolve headers when cross fetch is not available in environment

[vasco-santos]

This module supports using a custom fetch method, which is great when fetch is not supported in a given environment.

However, resolveHeaders function does not have into account that cross-fetch might be undefined. This was creating problems for me when using this module in a Cloudflare worker with the error: TypeError: Right-hand side of 'instanceof' is not an object.

This should have tests to avoid regressions, but at this point is quite complex to setup a test environment for this. I would suggest we can use something like https://github.com/hugomrdias/playwright-test and run tests in a service worker, but I left this for a follow up PR as I am not sure about how you would prefer to deal with this.

[faizan-ali]

Can we merge these changes? Our setup uses fetch-vcr which stubs out cross-fetch for a custom implementation of fetch globally through Jest - this modul's assumption of cross-fetch is breaking our testing setup.

[jasonkuhrt]

LGTM