Add test and bugfix to include an array of string

lib/active_model/serializer/include_tree.rb

@@ -52,7 +52,7 @@ module ActiveModel
               hash[key] = include_args_to_hash(value)
             end
           when Array
-            included.reduce({}) { |a, e| a.merge!(include_args_to_hash(e)) }
+            included.reduce({}) { |a, e| a.deep_merge!(include_args_to_hash(e)) }
           when String
             include_string_to_hash(included)
           else

test/include_tree/include_args_to_hash_test.rb

@@ -44,6 +44,19 @@ module ActiveModel
 
             assert_equal(expected, actual)
           end
+
+          def test_array_of_string
+            expected = {
+              comments: { author: {}, attachment: {} }
+            }
+            input = [
+              'comments.author',
+              'comments.attachment'
+            ]
+            actual = Parsing.include_args_to_hash(input)
+
+            assert_equal(expected, actual)
+          end
         end
       end
     end