Deprecate "args_for_retry" instead of removing it

Preserve backwards compatibility for gem-users that have overridden the
"args_for_retry" method.

Also, ensure that the tests for "args_for_retry", "retry_args" and
"retry_args_for_exception" are not overcomplicated, all we care about is
that they are hit.
My thought was, if we remove the base implementation of `args_for_retry` it will give us an easy, and clever, means to slip in a deprecation warning. By letting `args_for_retry` take precedence in the `retry_args` implementation we get the best of both worlds (we preserve backwards compatibility and are still able migrate our [internal] code to the new method).

With the next *minor* version increase, we should probably no longer support `args_for_retry`. That being said, I don't think there is any reason to add a method back, just to mark it deprecated (in this revision) only to remove it w/ the next minor version bump.

Going to sit on this overnight, review it at least once more and pull the trigger tomorrow. Thanks again, as always @lantins!