Make sure Array#to_sentence always returns a String

activesupport/lib/active_support/core_ext/array/conversions.rb

@@ -74,7 +74,7 @@ def to_sentence(options = {})
     when 0
       ''
     when 1
-      self[0].to_s.dup
+      "#{self[0]}"
     when 2
       "#{self[0]}#{options[:two_words_connector]}#{self[1]}"
     else

activesupport/test/core_ext/array/conversions_test.rb

@@ -60,6 +60,12 @@ def test_with_invalid_options
 
     assert_equal exception.message, "Unknown key: :passing. Valid keys are: :words_connector, :two_words_connector, :last_word_connector, :locale"
   end
+
+  def test_always_returns_string
+    assert_instance_of String, [ActiveSupport::SafeBuffer.new('one')].to_sentence
+    assert_instance_of String, [ActiveSupport::SafeBuffer.new('one'), 'two'].to_sentence
+    assert_instance_of String, [ActiveSupport::SafeBuffer.new('one'), 'two', 'three'].to_sentence
+  end
 end
 
 class ToSTest < ActiveSupport::TestCase