Remove string-key restriction for OrderedListInterface#toMap

[boesing]

There is a restriction in OrderedListInterface#toMap as of now so that we can actually prevent from creating another OrderedListInterface hidden in a MapInterface.

The main reason all of this exists is that there are issues with PHP type juggling in array keys and JSON responses:

https://3v4l.org/Y2ld5
https://3v4l.org/eI0Yj

My idea would be to actually remove the limitation of having integer keys but we should still prevent lists. Not quite sure how to do that but thats a problem for future max.