This change allows default responses to be rebuilt in client APIs as necessary. Certain use cases would be wrapping up errors in an external error type as well as modifying the data payload for a downstream response handler.

This class was never intended to be subclassable. It was incorrectly made open and should have always been public instead. Proxying can be done at the app levels using protocol rather than subclassing.

Request is actually DownloadRequest.

The AdaptError is only for internal use and should never be sent back to any public APIs such as the retrier. Previously, the AdaptError was being incorrectly sent to the retrier instead of the underlying error. This commit fixes the issue and also adds tests to ensure this is working properly.

Fixes issue where request property would always return `nil` if the url request was not used to create a URLSessionTask. This can commonly happen when an Error is thrown in an adapter.

It appears that in iOS 10 and macOS 10.12 that the root certificate is no longer included as part of the server trust object. The root certificate is still included in tvOS 10.0.