Fixes #15671 reflection strings now work as keys

This was fixed on master by #14675 but could not be backported because when the branch was made `_reflect_on_association` didn't exist so grabbing those changes would leave `_reflection_on_association` with the same issue.
 
This fixes #15671 - where string based keys don't work on nested associations.

Yup, good catch @matthewd :smile_cat: