Ensure Array#to_sentence does not modify given hash

Also simplify I18n logic for Array#to_sentence, doing only one lookup
for all keys and using merge!, instead of one lookup for each option key.

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

@@ -1,6 +1,5 @@
 require 'active_support/xml_mini'
 require 'active_support/core_ext/hash/keys'
-require 'active_support/core_ext/hash/reverse_merge'
 require 'active_support/core_ext/string/inflections'
 
 class Array
@@ -62,14 +61,10 @@ def to_sentence(options = {})
       :last_word_connector => ', and '
     }
     if defined?(I18n)
-      namespace = 'support.array.'
-      default_connectors.each_key do |name|
-        i18n_key = (namespace + name.to_s).to_sym
-        default_connectors[name] = I18n.translate i18n_key, :locale => options[:locale]
-      end
+      i18n_connectors = I18n.translate(:'support.array', locale: options[:locale], default: {})
+      default_connectors.merge!(i18n_connectors)
     end
-
-    options.reverse_merge! default_connectors
+    options = default_connectors.merge!(options)
 
     case length
     when 0

activesupport/test/core_ext/array_ext_test.rb

@@ -90,6 +90,12 @@ def test_one_element_not_same_object
   def test_one_non_string_element
     assert_equal '1', [1].to_sentence
   end
+
+  def test_does_not_modify_given_hash
+    options = { words_connector: ' ' }
+    assert_equal "one two, and three", ['one', 'two', 'three'].to_sentence(options)
+    assert_equal({ words_connector: ' ' }, options)
+  end
 end
 
 class ArrayExtToSTests < ActiveSupport::TestCase

activesupport/test/i18n_test.rb

@@ -97,4 +97,9 @@ def test_to_sentence
     I18n.backend.store_translations 'en', :support => { :array => { :two_words_connector => default_two_words_connector } }
     I18n.backend.store_translations 'en', :support => { :array => { :last_word_connector => default_last_word_connector } }
   end
+
+  def test_to_sentence_with_empty_i18n_store
+    I18n.backend.store_translations 'empty', {}
+    assert_equal 'a, b, and c', %w[a b c].to_sentence(locale: 'empty')
+  end
 end