Add a test case for preloading through association with implicit source

If `reflection_scope.where_clause` is not empty, `through_scope` should
be joined the source association. But if the through association doesn't
have explicit `:source`, `options[:source]` will be nil and
`scope.includes_values` will also be empty. It should use
`source_reflection.name` rather than `options[:source]`.

Fixed by a26cff3c.

Fixes #11078.
Fixes #26129.
Closes #14312.
Closes #29155.
Closes #29841.

activerecord/test/cases/associations/eager_test.rb

@@ -530,6 +530,14 @@ def test_eager_with_has_many_through_an_sti_join_model
     assert_equal [comments(:does_it_hurt)], assert_no_queries { author.special_post_comments }
   end
 
+  def test_preloading_has_many_through_with_implicit_source
+    authors = Author.includes(:very_special_comments).to_a
+    assert_no_queries do
+      special_comment_authors = authors.map { |author| [author.name, author.very_special_comments.size] }
+      assert_equal [["David", 1], ["Mary", 0], ["Bob", 0]], special_comment_authors
+    end
+  end
+
   def test_eager_with_has_many_through_an_sti_join_model_with_conditions_on_both
     author = Author.all.merge!(includes: :special_nonexistent_post_comments, order: "authors.id").first
     assert_equal [], author.special_nonexistent_post_comments