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.
Okay, I can see how this is working.

I wonder if its 'too clever' rather then just having a method for `args_for_retry` that calls `retry_args`? then we can output a warning and mark it as depreciated in the yardoc's?

The main thing is the user is informed about the breaking change, I guess how its done isn't so much of a problem.

Will leave it to your discretion @jzaleski :)