Fix Messages::Metadata#fresh? to handle parse_json_times = true

When `ActiveSupport.parse_json_times` is `true`,
`ActiveSupport::Messages::Medata.verify` will fail with a `TypeError` as
it does not pass a `String` to `Time.iso8601` as is expected.

This leads to hard-failure on retrieval of signed cookies with expiries.

activesupport/CHANGELOG.md

+*   Update `ActiveSupport::Messages::Metadata#fresh?` to work for cookies with expiry set when
+    `ActiveSupport.parse_json_times = true`.
+
+    *Christian Gregg*
+
 ## Rails 6.0.2 (December 13, 2019) ##
 
 *   Eager load translations during initialization.

activesupport/lib/active_support/messages/metadata.rb

@@ -64,7 +64,7 @@ def match?(purpose)
         end
 
         def fresh?
-          @expires_at.nil? || Time.now.utc < Time.iso8601(@expires_at)
+          @expires_at.nil? || Time.now.utc < @expires_at
         end
     end
   end

activesupport/test/message_verifier_test.rb

@@ -61,6 +61,15 @@ def test_alternative_serialization_method
     ActiveSupport.use_standard_json_time_format = prev
   end
 
+  def test_verify_with_parse_json_times
+    previous = [ ActiveSupport.parse_json_times, Time.zone ]
+    ActiveSupport.parse_json_times, Time.zone = true, "UTC"
+
+    assert_equal "hi", @verifier.verify(@verifier.generate("hi", expires_at: Time.now.utc + 10))
+  ensure
+    ActiveSupport.parse_json_times, Time.zone = previous
+  end
+
   def test_raise_error_when_argument_class_is_not_loaded
     # To generate the valid message below:
     #