Improvement for remember step

In this case if it is a wrong value in `upper_lower`, then nothing will be stored. You should either store the value or raise `NotImplementedError`:

```
def _store_with_case_option(...):
    if upper_lower:
        if 'lower' in upper_lower:
            value = value.lower()
        elif 'upper' in upper_lower:
            value = value.upper()
        # not necessary, keep it if you want to signal
        # that the passed value is wrong
        else:
            raise NotImplementedError('%s is not allowed' % upper_lower)
    world.stored_values[key] = value