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.

@matthewd @arthurnn I thought changing that was weird. But when I added that last `name.to_s` that I missed THAT test started failing which is what led me to look at master again. Perhaps that test and that last name should be switched back (all the tests pass in that case).

If that works I'll re-push those changes and open a new one to fix master as well (tests pass there too with those updates). Let me know your preferences here.