Ensure AR #second, #third, etc. finders work through associations

This PR addresses two regressions introduced in cafe31a078 (documented in #13783.) where newly created finder methods `#second`, `#third`, `#forth`, and `#fifth` caused a NoMethodError error on reload associations and where we were pulling the wrong element out of cached associations.

Examples:

```ruby

  some_book.authors.reload.second

  # Before
  # => NoMethodError: undefined method 'first' for nil:NilClass

  # After
  # => #<Author id: 2, name: "Sally Second", ...>

  some_book.first.authors.first
  some_book.first.authors.second

  # Before
  # => #<Author id: 1, name: "Freddy First", ...>
  # => #<Author id: 1, name: "Freddy First", ...>

  # After
  # => #<Author id: 1, name: "Freddy First", ...>
  # => #<Author id: 2, name: "Sally Second", ...>]

```