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.
There are a lot of redirect codes, not just `304`: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

If we want to support redirects, I think we should follow them, not just resolve with response body.