fix: honor custom retry in job.result()

[chalmerlowe]

The _AsyncJob.result() method was not correctly passing the right arguments to the superclass's result() method when the retry object was expected to be the same as the DEFAULT_RETRY object. Passing an empty dict was a way of simply letting the google.api_core handle things.

{} if retry is DEFAULT_RETRY else {"retry": retry}

When the retry object is instantiated and when the DEFAULT_RETRY object is instantiated, these two elements are separate objects. This means that because the is keyword checks for identity NOT equality that code path never evaluates to True and is not used as one would expect.

This leads to subtle bugs that this PR corrects by simply passing the retry object and relying on the google.api_core to understand what a DEFAULT_RETRY value is and managing it appropriately.

A new test case is added to verify that job.result() correctly handles both the default retry use case and a custom retry object use case.

Fixes #2210 🦕