_simplify_up with MergeAsOfIndexed results in "object has no attribute 'left_on'" error

[kapitalk26]

Line 528 in _merge.py reads:
left_on = _convert_to_list(self.left_on)
When self is a MergeAsOfIndexed object, this will fail since MergeAsofIndexed does not have a left_on attribute.

I don't have a simple way to reproduce yet. This came from our production application using multiple sequential merge_asof calls