Fix error handling as exceptions.

[askreet]

Goal

The tests for errors as exceptions are failing on master. As far as I could tell in my own work, they simply don't work. The issue is Faraday's middleware base class uses respond_to?(:on_complete) to determine if the hook should be called, but it is declared as private, meaning that will always be false.

Design

N/A.

Changeset

Remove private specifier for the on_complete handler.

Testing

The existing tests pass now.

[mattdyoung]

Thanks for the PR! We'll look at reviewing / merging when we can.