Test collection caching with callable cache key.

When people pass `cache: -> item { item.upcase }` they scope the collection
cache keys so the individual partial cache isn't reused.

Test that behavior.

actionpack/test/controller/caching_test.rb

@@ -390,6 +390,11 @@ def index_with_comment
     @customers = [Customer.new('david', 1)]
     render partial: 'customers/commented_customer', collection: @customers, as: :customer
   end
+
+  def index_with_callable_cache_key
+    @customers = [Customer.new('david', 1)]
+    render @customers, cache: -> customer { 'cached_david' }
+  end
 end
 
 class AutomaticCollectionCacheTest < ActionController::TestCase
@@ -430,6 +435,26 @@ def test_caching_works_with_beginning_comment
     get :index_with_comment
     assert_equal 1, @controller.partial_rendered_times
   end
+
+  def test_caching_with_callable_cache_key
+    get :index_with_callable_cache_key
+    assert_equal 1, @controller.partial_rendered_times
+    assert_select ':root', 'david, 1'
+
+    get :index_with_callable_cache_key
+    assert_equal 1, @controller.partial_rendered_times
+    assert_select ':root', 'david, 1'
+  end
+
+  def test_caching_mixing_callable_cache_key_and_automatic_caching
+    get :index
+    assert_equal 1, @controller.partial_rendered_times
+    assert_select ':root', 'david, 1'
+
+    get :index_with_callable_cache_key
+    assert_equal 1, @controller.partial_rendered_times, 'individual cache not reused with collection'
+    assert_select ':root', 'david, 1'
+  end
 end
 
 class FragmentCacheKeyTestController < CachingController