Add partial iteration variable to template keys

When rendering heterogeneous collection using `render @collection` or
`render partial: @collection`, the expected `<partial_name>_iteration`
variable is missing due to `find_template` not having the name of the
iteration variable included in its cache keys.

actionview/lib/action_view/renderer/partial_renderer.rb

@@ -458,7 +458,7 @@ module ActionView
           locals[counter]   = index
           locals[iteration] = partial_iteration
 
-          template = (cache[path] ||= find_template(path, keys + [as, counter]))
+          template = (cache[path] ||= find_template(path, keys + [as, counter, iteration]))
           content = template.render(view, locals)
           partial_iteration.iterate!
           content

actionview/test/fixtures/test/_partial_iteration_1.erb

@@ -0,0 +1 @@
+<%= defined?(partial_iteration_1_iteration) %>

actionview/test/fixtures/test/_partial_iteration_2.erb

@@ -0,0 +1 @@
+<%= defined?(partial_iteration_2_iteration) -%>

actionview/test/template/render_test.rb

@@ -301,6 +301,15 @@ module RenderTestCases
       @view.render(partial: "test/local_inspector", collection: [ Customer.new("mary") ])
   end
 
+  def test_render_partial_collection_with_different_partials_still_provides_partial_iteration
+    a = {}
+    b = {}
+    def a.to_partial_path; "test/partial_iteration_1"; end
+    def b.to_partial_path; "test/partial_iteration_2"; end
+
+    assert_equal "local-variable\nlocal-variable", @controller_view.render([a, b])
+  end
+
   def test_render_partial_with_empty_collection_should_return_nil
     assert_nil @view.render(partial: "test/customer", collection: [])
   end