This commit fixes two regressions introduced in cafe31a078 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:
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", ...>
Fixes#13783.
