[REVIEW REQUEST] IGNITE-11951 Improvements in JdkMarshaller

[Павлухин Иван]

Hi,

I made some small improvements in JdkMarshaller [1]. I will be happy
if someone reviews it. Changes are quite simple.

[1] https://issues.apache.org/jira/browse/IGNITE-11951

-- 
Best regards,
Ivan Pavlukhin