Revert "Further simplify singleton_class checking in class_attribute"

This reverts commit 520918aad9.

Reason: build failure

activesupport/lib/active_support/core_ext/class/attribute.rb

@@ -81,13 +81,21 @@ class Class
             define_method(:#{name}) { val }
           end
 
+          if singleton_class?
+            class_eval do
+              remove_possible_method(:#{name})
+              def #{name}
+                defined?(@#{name}) ? @#{name} : singleton_class.#{name}
+              end
+            end
+          end
           val
         end
 
         if instance_reader
           remove_possible_method :#{name}
           def #{name}
-            defined?(@#{name}) ? @#{name} : singleton_class.#{name}
+            defined?(@#{name}) ? @#{name} : self.class.#{name}
           end
 
           def #{name}?
@@ -99,4 +107,9 @@ class Class
       attr_writer name if instance_writer
     end
   end
+
+  private
+  def singleton_class?
+    !name || '' == name
+  end
 end