Make sure XMLHttpRequest#responseText is always a string

The [XMLHttpRequest Level 2](http://www.w3.org/TR/XMLHttpRequest2/) specification requires the `responseText` attribute to be a "text response entity body". Accoding to [the relevant section](http://www.w3.org/TR/XMLHttpRequest2/#text-response-entity-body) in the spec, this means that it must always be a string, regardless of the response's content type.

The commit of this pull request adjusts zombie.js to match this behavior. Previously, `responseText` would be a `Buffer` in such cases.