Default qualifier accepts all valid statuses.

Fixes #120.

Note that this fix accepts all 2xx responses (which are always defined as success) and a 304 response (which is defined as a success whenever certain headers are set).

This is a breaking change, and I advise at least a minor version upgrade.
Why not to make numerical comparison?

return `(response.status >== 200 && response.status < 300) || response.status === 304`