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.

@arthurnn #15210 and #15300 disagree on this being a symbol vs string. Should we be changing this?

If the point of the new `reflections` is to maintain the 4.0 interface, I guess we should stick with symbol... and do so in master as well. The stringification (of what is now `_reflections`) in #14675 was predicated on the fact that we could freely change it because it's internal.

Alternatively, there's always HWIA... :confused: