Named scopes shouldn't test equality using to_a if it's not an Array, this was...

Named scopes shouldn't test equality using to_a if it's not an Array, this was causing records to be loaded before they were needed.
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

activerecord/lib/active_record/named_scope.rb

@@ -167,7 +167,14 @@ def last(*args)
       end
 
       def ==(other)
-        other.respond_to?(:to_ary) ? to_a == other.to_a : false
+        case other
+        when Scope
+          to_sql == other.to_sql
+        when Relation
+          other == self
+        when Array
+          to_a == other.to_a
+        end
       end
 
       private

activerecord/test/cases/named_scope_test.rb

@@ -407,6 +407,12 @@ def test_index_on_named_scope
     assert_equal topics(:second), approved[0]
     assert approved.loaded?
   end
+
+  def test_nested_named_scopes_queries_size
+    assert_queries(1) do
+      Topic.approved.by_lifo.replied.written_before(Time.now).all
+    end
+  end
 end
 
 class DynamicScopeMatchTest < ActiveRecord::TestCase