Update WritingTests.md (#2686)
Update WritingTests.md (#2686)

https://github.com/wheresrhys/fetch-mock/issues/104

I wrangled with why the testing for 'isomorphic-fetch' and 'fetch-mock' didn't work before finding the above issue, and realizing that you must import 'isomorphic-fetch' as a global for 'fetch-mock' to override it. Wanted to update the docs so the next person doesn't have the same issue :)
gaearon/redux,CompuIves/redux,gaearon/redux,CompuIves/redux