fix step arguments with default value

[samuelspagl]

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The bug is mentioned in the issue #607. This is a sample implementation trying minimal invasive by implementing a new function that retrieves all arguments with default values.

Another solution would be to let get_args retrieve a dict with the arguments as keys and its values as values. But this would add two or three more changes I think, therefore I didn't implement this solution.

I'm really looking forward hearing from you.

Best Samuel