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.
Thanks for explaining your rational, I _think_ I get it :)

I've not personally dealt with the deprecation of methods within a widely used public API before, so I have no good (or bad) experiences on the subject to fall back upon.

I do agree wholly with the change (renaming the method), that makes perfect sense.

My underlying thought is we KISS (keep it simple stupid).

Because were making a breaking change to the API; we need to perform a major version bump when the method no-longer works.