Fail if collection type cannot be inferred with json adapter (#2210)

test/collection_serializer_test.rb

@@ -93,12 +93,16 @@ module ActiveModel
         resource = []
         resource.define_singleton_method(:name) { nil }
         serializer = collection_serializer.new(resource)
-        assert_nil serializer.json_key
+        assert_raise ArgumentError do
+          serializer.json_key
+        end
       end
 
       def test_json_key_with_resource_without_name_and_no_serializers
         serializer = collection_serializer.new([])
-        assert_nil serializer.json_key
+        assert_raise ArgumentError do
+          serializer.json_key
+        end
       end
 
       def test_json_key_with_empty_resources_with_serializer