Fix `thread_mattr_accessor` thread-local variable naming

The current implentation of `thread_mattr_accessor` is setting
differently-named thread variables when defining class and
instance writer methods, so the method isn't working as documented:

    Account.user = "DHH"
    Account.user # => "DHH"
    Account.new.user # => nil
    a = Account.new
    a.user = "ABC" # => "ABC"
    a.class.user # => "DHH"

At this point `:attr_Account_user` and `:attr_Class_user` thread-local
variables have been created. Modify the reader and writer methods to use
the class name instead of 'Class'.

activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb

@@ -47,7 +47,7 @@ def self.#{sym}
       unless options[:instance_reader] == false || options[:instance_accessor] == false
         class_eval(<<-EOS, __FILE__, __LINE__ + 1)
           def #{sym}
-            Thread.current[:"attr_#{self.class.name}_#{sym}"]
+            Thread.current[:"attr_#{name}_#{sym}"]
           end
         EOS
       end
@@ -86,7 +86,7 @@ def self.#{sym}=(obj)
       unless options[:instance_writer] == false || options[:instance_accessor] == false
         class_eval(<<-EOS, __FILE__, __LINE__ + 1)
           def #{sym}=(obj)
-            Thread.current[:"attr_#{self.class.name}_#{sym}"] = obj
+            Thread.current[:"attr_#{name}_#{sym}"] = obj
           end
         EOS
       end

activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb

@@ -106,4 +106,10 @@ def test_should_raise_name_error_if_attribute_name_is_invalid
     end
     assert_equal "invalid attribute name: 2valid_part", exception.message
   end
+
+  def test_should_return_same_value_by_class_or_instance_accessor
+    @class.foo = 'fries'
+
+    assert_equal @class.foo, @object.foo
+  end
 end