add AS::SafeBuffer#clone_empty

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

@@ -101,6 +101,12 @@ def initialize_copy(other)
       @dirty = other.dirty?
     end
 
+    def clone_empty
+      new_safe_buffer = self[0, 0]
+      new_safe_buffer.instance_variable_set(:@dirty, @dirty)
+      new_safe_buffer
+    end
+
     def concat(value)
       if dirty? || value.html_safe?
         super(value)

activesupport/test/safe_buffer_test.rb

@@ -102,4 +102,13 @@ def setup
       @buffer.safe_concat "BUSTED"
     end
   end
+
+  test "clone_empty returns an empty buffer" do
+    assert_equal '', ActiveSupport::SafeBuffer.new('foo').clone_empty
+  end
+
+  test "clone_empty keeps the original dirtyness" do
+    assert @buffer.clone_empty.html_safe?
+    assert !@buffer.gsub!('', '').clone_empty.html_safe?
+  end
 end