Remove additional header checks
The additional header checks were added previously while
investigating another issue, but the issue has been since
resolved. The additional checks were left in place to
ensure the issue did not reoccur but we've had zero hits
since. They can be safely removed now.
Use a shared Fetcher instance for ServiceWorkerGlobalScope requests
We were creating a new Fetcher instance for every request
despite the fact that the Fetcher itself was stateless and
not storing any request-specific data. This changes it so
that all requests to a given ServiceWorkerGlobalScope instance
share a single default Fetcher instance.