Superagent does not always produce a `body`. See
http://visionmedia.github.io/superagent/ for details. When it
doesn't, we should deserialize the raw `response.text` rather
than returning `null`. Currently, the JS client always returns
`null` when the return type is String! This commit fixes
that.
The `constructFromObject` factory methods should be class methods
(or "static" methods), not instance methods.
With this commit, ApiClient no longer calls the model constructors
directly. Instead, it calls the new static factory method to get the
new instance. If there is no data on the top level, null is returned.
It is still possible for users to call the model constructors
directly, of course.
