accept application/json

Fetching a tarball from GitHub returns a 415 if the request doesn't accept `application/json`.

Writing the test took longer than writing the fix :)

fixes https://github.com/npm/npm/issues/6729
Could very well be the name of the test too, I guess. Actually now that I think about it, `application/json` might be necessary because it's the `api.github.com` subdomain that's we're emulating, and I guess JSON is needed for everything there.