Fix random test failure of test_create_resets_cached_counters

- In earlier commit, I removed setting id manually for readers fixtures
  but that did not fix the randomly failing
  test_create_resets_cached_counters
  from has_many_associations tests.
- Because the problem was with the `person_id` of the readers. As it is
  set to 1 in fixtures, if a post gets created with id 1 then that post
  automatically has 2 readers.
- Fixed by removing the person_id.

Author: prathamesh-sonpatki

activerecord/test/fixtures/readers.yml

@@ -1,9 +1,9 @@
 michael_welcome:
   post_id: 1
-  person_id: 1
+  person: michael
   first_post_id: 2
 
 michael_authorless:
   post_id: 3
-  person_id: 1
+  person: michael
   first_post_id: 3