Fix eager_loading? when ordering with Symbol

`eager_loading?` is triggered correctly when using `order` with symbols.

```ruby
scope = Post.includes(:comments).order(:"comments.label")
=> true
```

activerecord/CHANGELOG.md

@@ -1,3 +1,14 @@
+*   Fix `eager_loading?` when ordering with `Symbol`
+
+    `eager_loading?` is triggered correctly when using `order` with symbols.
+
+    ```ruby
+    scope = Post.includes(:comments).order(:"comments.label")
+    => true
+    ```
+
+    *Jacopo Beschi*
+
 *   Two change tracking methods are added for `belongs_to` associations.
 
     The `association_changed?` method (assuming an association named `:association`) returns true

activerecord/lib/active_record/relation/query_methods.rb

@@ -1550,7 +1550,7 @@ module ActiveRecord
       def column_references(order_args)
         references = order_args.flat_map do |arg|
           case arg
-          when String
+          when String, Symbol
             arg
           when Hash
             arg.keys

activerecord/test/cases/relations_test.rb

@@ -1714,6 +1714,16 @@ class RelationTest < ActiveRecord::TestCase
     assert_not_predicate scope, :eager_loading?
   end
 
+  def test_order_triggers_eager_loading_when_ordering_using_symbols
+    scope = Post.includes(:comments).order(:"comments.label")
+    assert_predicate scope, :eager_loading?
+  end
+
+  def test_order_doesnt_trigger_eager_loading_when_ordering_using_owner_table_and_symbols
+    scope = Post.includes(:comments).order(:"posts.title")
+    assert_not_predicate scope, :eager_loading?
+  end
+
   def test_order_triggers_eager_loading_when_ordering_using_hash_syntax
     scope = Post.includes(:comments).order({ "comments.label": :ASC })
     assert_predicate scope, :eager_loading?