Fix `thread_mattr_accessor` share variable superclass with subclass

The current implementation of `thread_mattr_accessor` set variable
sharing superclass with subclass. So the method doesn't work as documented.

Precondition

    class Account
      thread_mattr_accessor :user
    end

    class Customer < Account
    end

    Account.user = "DHH"
    Account.user  #=> "DHH"
    Customer.user = "Rafael"
    Customer.user # => "Rafael"

Documented behavior

    Account.user  # => "DHH"

Actual behavior

    Account.user  # => "Rafael"

Current implementation set variable statically likes `Thread[:attr_Account_user]`,
and customer also use it.

Make variable name dynamic to use own thread-local variable.

activesupport/CHANGELOG.md

+*   Fix `thread_mattr_accessor` share variable superclass with subclass
+
+    The current implementation of `thread_mattr_accessor` set variable
+    sharing superclass with subclass. So the method doesn't work as documented.
+
+    *Shinichi Maeshima*
+
 *   Since weeks are no longer converted to days, add `:weeks` to the list of
     parts that `ActiveSupport::TimeWithZone` will recognize as possibly being
     of variable duration to take account of DST transitions.

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

@@ -41,14 +41,14 @@ def thread_mattr_reader(*syms)
       raise NameError.new("invalid attribute name: #{sym}") unless /^[_A-Za-z]\w*$/.match?(sym)
       class_eval(<<-EOS, __FILE__, __LINE__ + 1)
         def self.#{sym}
-          Thread.current[:"attr_#{name}_#{sym}"]
+          Thread.current["attr_"+ name + "_#{sym}"]
         end
       EOS
 
       unless options[:instance_reader] == false || options[:instance_accessor] == false
         class_eval(<<-EOS, __FILE__, __LINE__ + 1)
           def #{sym}
-            Thread.current[:"attr_#{name}_#{sym}"]
+            Thread.current["attr_"+ self.class.name + "_#{sym}"]
           end
         EOS
       end
@@ -80,14 +80,14 @@ def thread_mattr_writer(*syms)
       raise NameError.new("invalid attribute name: #{sym}") unless /^[_A-Za-z]\w*$/.match?(sym)
       class_eval(<<-EOS, __FILE__, __LINE__ + 1)
         def self.#{sym}=(obj)
-          Thread.current[:"attr_#{name}_#{sym}"] = obj
+          Thread.current["attr_"+ name + "_#{sym}"] = obj
         end
       EOS
 
       unless options[:instance_writer] == false || options[:instance_accessor] == false
         class_eval(<<-EOS, __FILE__, __LINE__ + 1)
           def #{sym}=(obj)
-            Thread.current[:"attr_#{name}_#{sym}"] = obj
+            Thread.current["attr_"+ self.class.name + "_#{sym}"] = obj
           end
         EOS
       end

activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb

@@ -8,6 +8,16 @@ def setup
       thread_mattr_accessor :bar,  instance_writer: false
       thread_mattr_reader   :shaq, instance_reader: false
       thread_mattr_accessor :camp, instance_accessor: false
+
+      def self.name
+        'MyClass'
+      end
+    end
+
+    @subclass = Class.new(@class) do
+      def self.name
+        'SubMyClass'
+      end
     end
 
     @object = @class.new
@@ -65,15 +75,15 @@ def test_values_should_not_bleed_between_threads
     threads << Thread.new do
       @class.foo = 'other things'
       sleep 1
-      assert_equal 'other things', @class.foo      
+      assert_equal 'other things', @class.foo
     end
-    
+
     threads << Thread.new do
       @class.foo = 'really other things'
       sleep 1
-      assert_equal 'really other things', @class.foo      
+      assert_equal 'really other things', @class.foo
     end
-    
+
     threads.each { |t| t.join }
   end
 
@@ -112,4 +122,14 @@ def test_should_return_same_value_by_class_or_instance_accessor
 
     assert_equal @class.foo, @object.foo
   end
+
+  def test_should_not_affect_superclass_if_subclass_set_value
+    @class.foo = 'super'
+    assert_equal @class.foo, 'super'
+    assert_nil @subclass.foo
+
+    @subclass.foo = 'sub'
+    assert_equal @class.foo, 'super'
+    assert_equal @subclass.foo, 'sub'
+  end
 end