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.
If its been removed, we cant mark it as deprecated, but then if its removed entirely were not keeping backwards computability?

I'll look again tomorrow when I've not had 1-8 beers (exact quantity is withheld ;))