make sure we play nicely when syck is activated

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

 require 'erb'
 require 'active_support/core_ext/kernel/singleton_class'
-require 'active_support/core_ext/yaml'
 
 class ERB
   module Util
@@ -102,10 +101,14 @@ def to_s
       self
     end
 
-    unless defined?(Psych)
-      def to_yaml(*args)
-        to_str.to_yaml(*args)
-      end
+    def encode_with(coder)
+      coder.represent_scalar nil, to_str
+    end
+
+    def to_yaml(*args)
+      return super() if defined?(YAML::ENGINE) && !YAML::ENGINE.syck?
+
+      to_str.to_yaml(*args)
     end
   end
 end

activesupport/test/safe_buffer_test.rb

 require 'abstract_unit'
+begin
+  require 'psych'
+rescue LoadError
+end
+
+require 'yaml'
 
 class SafeBufferTest < ActiveSupport::TestCase
   def setup
@@ -38,4 +44,20 @@ def setup
     new_buffer = @buffer.to_s
     assert_equal ActiveSupport::SafeBuffer, new_buffer.class
   end
+
+  def test_to_yaml
+    str  = 'hello!'
+    buf  = ActiveSupport::SafeBuffer.new str
+    yaml = buf.to_yaml
+
+    assert_match(/^--- #{str}/, yaml)
+    assert_equal 'hello!', YAML.load(yaml)
+  end
+
+  def test_nested
+    str  = 'hello!'
+    data = { 'str' => ActiveSupport::SafeBuffer.new(str) }
+    yaml = YAML.dump data
+    assert_equal({'str' => str}, YAML.load(yaml))
+  end
 end