Fix cancellation for GraphQL requests

Created by: eseliger

Due to a bug in the fromFetch method, cancellation is not working when using response.json. That lead to repeatWhen for example not correctly unsubscribing, basically fetching forever even after a component was unmounted.

This is also currently happening on pages for repos that are having a cloning status, after leaving the page, they will be fetched until it's done cloning.

(see network tab) Before:

After: