Thanks for the PR! Looks good! Cheers! 🍻 

`URL.appendPathComponent` is mutating; the example is set up to use the generating version `URL.appendingPathComponent`.

standard library equivalent in its place.

* Update Travis config to Xcode 8.3.

This also removes the explicit SDK settings, as they should be
controlled through the project instead. It’s also one less setting to
keep up-to-date.

* Limit branch building to master.

Thanks for the PR! It appears `httpbin.org` made this change without us realizing it.

The example code in the "Modifying the Session Configuration" section was calling the `defaultHTTPHeaders` property as an instance property rather than static.

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.