Mixin `CollectionProxy::DelegateExtending` after `ClassSpecificRelation`

`ClassSpecificRelation` has `method_missing` and the `method_missing` is
called first. if an associated class has the missing method in a
relation, never reach to the `method_missing` in the `CollectionProxy`.
I extracted `DelegateExtending` and included it to the delegate class
that including `ClassSpecificRelation` to fix the issue.

Fixes https://github.com/rails/rails/pull/28246#issuecomment-296033784.

activerecord/lib/active_record/associations/collection_proxy.rb

@@ -1121,6 +1121,19 @@ def reset
 
       delegate(*delegate_methods, to: :scope)
 
+      module DelegateExtending # :nodoc:
+        private
+          def method_missing(method, *args, &block)
+            extending_values = association_scope.extending_values
+            if extending_values.any? && (extending_values - self.class.included_modules).any?
+              self.class.include(*extending_values)
+              public_send(method, *args, &block)
+            else
+              super
+            end
+          end
+      end
+
       private
 
         def find_nth_with_limit(index, limit)
@@ -1141,21 +1154,16 @@ def find_from_target?
           @association.find_from_target?
         end
 
+        def association_scope
+          @association.association_scope
+        end
+
         def exec_queries
           load_target
         end
 
         def respond_to_missing?(method, _)
-          scope.respond_to?(method) || super
-        end
-
-        def method_missing(method, *args, &block)
-          if scope.respond_to?(method) && scope.extending_values.any?
-            extend(*scope.extending_values)
-            public_send(method, *args, &block)
-          else
-            super
-          end
+          association_scope.respond_to?(method) || super
         end
     end
   end

activerecord/lib/active_record/relation/delegation.rb

@@ -25,6 +25,8 @@ def initialize_relation_delegate_cache
 
       def inherited(child_class)
         child_class.initialize_relation_delegate_cache
+        delegate = child_class.relation_delegate_class(ActiveRecord::Associations::CollectionProxy)
+        delegate.include ActiveRecord::Associations::CollectionProxy::DelegateExtending
         super
       end
     end

activerecord/test/models/comment.rb

@@ -19,6 +19,11 @@ class Comment < ActiveRecord::Base
   has_many :children, class_name: "Comment", foreign_key: :parent_id
   belongs_to :parent, class_name: "Comment", counter_cache: :children_count
 
+  # Should not be called if extending modules that having the method exists on an association.
+  def self.greeting
+    raise
+  end
+
   def self.what_are_you
     "a comment..."
   end