Support assignments for composite key has_many through associations

Given a has_many through association with a composite foreign key, like:
```ruby
class BlogPost < ApplicationRecord
  query_constraints :blog_id, :id

  has_many :blog_post_tags, query_constraints: [:blog_id, :blog_post_id]
  has_many :tags, through: :blog_post_tags
end

class BlogPostTag < ApplicationRecord
  belongs_to :blog_post, query_constraints: [:blog_id, :blog_post_id]
  belongs_to :tag, query_constraints: [:blog_id, :tag_id]
end

class Tag < ApplicationRecord
  query_constraints :blog_id, :id

  has_many :blog_post_tags, query_constraints: [:blog_id, :tag_id]
  has_many :blog_posts, through: :blog_post_tags
end
```

It should be possible to assign tags to a blog post:
```ruby
blog_post = BlogPost.create!(title: "Hello world")
blog_post.tags << Tag.new(name: "ruby")
```

Author: nvasilevski

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -69,9 +69,12 @@ def build_through_record(record)
 
         def through_scope_attributes
           scope = through_scope || self.scope
-          scope.where_values_hash(through_association.reflection.name.to_s).
-            except!(through_association.reflection.foreign_key,
-                    through_association.reflection.klass.inheritance_column)
+          attributes = scope.where_values_hash(through_association.reflection.name.to_s)
+          except_keys = [
+            *Array(through_association.reflection.foreign_key),
+            through_association.reflection.klass.inheritance_column
+          ]
+          attributes.except!(*except_keys)
         end
 
         def save_through_record(record)

activerecord/test/cases/associations_test.rb

@@ -231,6 +231,27 @@ def test_assign_composite_foreign_key_belongs_to_association_with_autosave
     assert_equal(another_blog.id, comment.blog_id)
     assert_equal(comment.blog_post_id, blog_post.id)
   end
+
+  def test_append_composite_has_many_through_association
+    blog_post = sharded_blog_posts(:great_post_blog_one)
+    tag = Sharded::Tag.new(name: "Ruby on Rails", blog_id: blog_post.blog_id)
+    tag.save
+
+    blog_post.tags << tag
+
+    assert_includes(blog_post.reload.tags, tag)
+    assert_predicate Sharded::BlogPostTag.where(blog_post_id: blog_post.id, blog_id: blog_post.blog_id, tag_id: tag.id), :exists?
+  end
+
+  def test_append_composite_has_many_through_association_with_autosave
+    blog_post = sharded_blog_posts(:great_post_blog_one)
+    tag = Sharded::Tag.new(name: "Ruby on Rails", blog_id: blog_post.blog_id)
+
+    blog_post.tags << tag
+
+    assert_includes(blog_post.reload.tags, tag)
+    assert_predicate Sharded::BlogPostTag.where(blog_post_id: blog_post.id, blog_id: blog_post.blog_id, tag_id: tag.id), :exists?
+  end
 end
 
 class AssociationProxyTest < ActiveRecord::TestCase