Run GitHub/fetch tests against this polyfill

I did try putting together a PR that runs all tests at [github/fetch](https://github.com/github/fetch) but got overwhelmed by amount of work required. Basically it should be done via Git submodules and a shell/node script that swaps the implementation in the submodule with dist of this project.

Doing most of things manually I saw a few tests failing.

Start with the github/fetch test suite and this project's dist output, then review the Git-submodule and shell/Node workflow described in the issue. Done means an automated script replaces github/fetch's implementation with this project's dist, runs the upstream tests, and records the observed failures.

Scritto dal modello di indicizzazione a partire dal testo della issue.