RFR [15] 8238763: ObjectInputStream readUnshared method handling of Records

[Chris Hegarty]

This is a small change to ObjectInputStream, for 8238763 [1], to correctly 
handle readUnshared for records.

Webrev:
  https://cr.openjdk.java.net/~chegar/8238763/webrev.00/

-Chris.

[1] https://bugs.openjdk.java.net/browse/JDK-8238763