Fix tests in the master branch

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