Resolving #283 user warning when delay_after_gen is not in use

[greenLeopard]

I've made three small changes to remove the UserWarning without suppressing all warnings (#283 ).

1. In the constructor, set the default value for delay_after_gen to None.
2. In the validation step, if the incoming argument is None, set self.delay_after_gen to None.
3. Before calling time.sleep(self.delay_after_gen) just check whether it's None.

Although, the alternative, since it's being deprecated, is to remove all references to delay_after_gen. I'll be happy to do that if it's your preference, then I'll resubmit the PR.