delete vulnerable AS::SafeBuffer#[]

activesupport/lib/active_support/core_ext/string/output_safety.rb

@@ -103,12 +103,6 @@ def initialize
       end
     end
 
-    def[](*args)
-      new_safe_buffer = super
-      new_safe_buffer.instance_eval { @dirty = false }
-      new_safe_buffer
-    end
-
     def safe_concat(value)
       raise SafeConcatError if dirty?
       original_concat(value)

activesupport/test/safe_buffer_test.rb

@@ -113,12 +113,6 @@ def test_titleize
     assert_kind_of NilClass, @buffer.slice("chipchop")
   end
 
-  test "Should initialize @dirty to false for new instance when sliced" do
-    dirty = @buffer[0,0].send(:dirty?)
-    assert_not_nil dirty
-    assert !dirty
-  end
-
   test "clone_empty returns an empty buffer" do
     assert_equal '', ActiveSupport::SafeBuffer.new('foo').clone_empty
   end