Fix expanding cache key for single element arrays

In short:
expand_cache_key(element) should not equal expand_cache_key([element])

This way a fragment cache key for an index page with only a single
element in the collection is different than a fragment cache for a
typical show page for that element.

activesupport/lib/active_support/cache.rb

@@ -90,7 +90,7 @@ def expand_cache_key(key, namespace = nil)
       def retrieve_cache_key(key)
         case
         when key.respond_to?(:cache_key) then key.cache_key
-        when key.is_a?(Array)            then key.map { |element| retrieve_cache_key(element) }.to_param
+        when key.is_a?(Array)            then ['Array', *key.map { |element| retrieve_cache_key(element) }].to_param
         else                                  key.to_param
         end.to_s
       end

activesupport/test/caching_test.rb

@@ -4,19 +4,19 @@
 
 class CacheKeyTest < ActiveSupport::TestCase
   def test_expand_cache_key
-    assert_equal '1/2/true', ActiveSupport::Cache.expand_cache_key([1, '2', true])
-    assert_equal 'name/1/2/true', ActiveSupport::Cache.expand_cache_key([1, '2', true], :name)
+    assert_equal 'Array/1/2/true', ActiveSupport::Cache.expand_cache_key([1, '2', true])
+    assert_equal 'name/Array/1/2/true', ActiveSupport::Cache.expand_cache_key([1, '2', true], :name)
   end
 
   def test_expand_cache_key_with_rails_cache_id
     begin
       ENV['RAILS_CACHE_ID'] = 'c99'
       assert_equal 'c99/foo', ActiveSupport::Cache.expand_cache_key(:foo)
-      assert_equal 'c99/foo', ActiveSupport::Cache.expand_cache_key([:foo])
-      assert_equal 'c99/foo/bar', ActiveSupport::Cache.expand_cache_key([:foo, :bar])
+      assert_equal 'c99/Array/foo', ActiveSupport::Cache.expand_cache_key([:foo])
+      assert_equal 'c99/Array/foo/bar', ActiveSupport::Cache.expand_cache_key([:foo, :bar])
       assert_equal 'nm/c99/foo', ActiveSupport::Cache.expand_cache_key(:foo, :nm)
-      assert_equal 'nm/c99/foo', ActiveSupport::Cache.expand_cache_key([:foo], :nm)
-      assert_equal 'nm/c99/foo/bar', ActiveSupport::Cache.expand_cache_key([:foo, :bar], :nm)
+      assert_equal 'nm/c99/Array/foo', ActiveSupport::Cache.expand_cache_key([:foo], :nm)
+      assert_equal 'nm/c99/Array/foo/bar', ActiveSupport::Cache.expand_cache_key([:foo, :bar], :nm)
     ensure
       ENV['RAILS_CACHE_ID'] = nil
     end
@@ -55,7 +55,7 @@ def test_expand_cache_key_array_with_something_that_responds_to_cache_key
     def key.cache_key
       :foo_key
     end
-    assert_equal 'foo_key', ActiveSupport::Cache.expand_cache_key([key])
+    assert_equal 'Array/foo_key', ActiveSupport::Cache.expand_cache_key([key])
   end
 
   def test_expand_cache_key_of_nil
@@ -69,6 +69,14 @@ def test_expand_cache_key_of_false
   def test_expand_cache_key_of_true
     assert_equal 'true', ActiveSupport::Cache.expand_cache_key(true)
   end
+
+  def test_expand_cache_key_of_one_element_array_different_than_key_of_element
+    element = 'foo'
+    array = [element]
+    element_cache_key = ActiveSupport::Cache.expand_cache_key(element)
+    array_cache_key = ActiveSupport::Cache.expand_cache_key(array)
+    assert_not_equal element_cache_key, array_cache_key
+  end
 end
 
 class CacheStoreSettingTest < ActiveSupport::TestCase