Fix tests in the master branch

The following things were broken due to changes in Wreck:

* Wreck.read now takes `null` as its second argument
* The tests should use `Wreck.toReadableStream`, because on a normal `Stream`, `Wreck.read` never completes

Things that were broken in the tests, since the code changed:
* The tests should put headers on mocked the response object, because `addCorsAndBearerToken` expects this to exist

Things that were actually broken in the code:
* The request headers are an object, not an array, so an extra `Object.keys` call is necessary