Freeze association foreign keys to reduce allocations

The string returned here will ultimately get used as a key of a hash in
the attribute set once the attributes are being built. When you give a
non-frozen string to `Hash#[]`, it will be duped. Be freezing we can
significantly reduce the number of times we end up allocating
`"user_id"`

This does not include any additional tests, as this should not have any
public facing implications. If you are mutating the result of
`Reflection#foreign_key`, please stop.

activerecord/lib/active_record/reflection.rb

@@ -371,7 +371,7 @@ module ActiveRecord
       end
 
       def foreign_key
-        @foreign_key ||= options[:foreign_key] || derive_foreign_key
+        @foreign_key ||= options[:foreign_key] || derive_foreign_key.freeze
       end
 
       def association_foreign_key