RFR 8233048: WeekFields.ISO is not a singleton

[naoto.sato at oracle.com]

Hi,

Please review the fix to the following issue:

https://bugs.openjdk.java.net/browse/JDK-8233048

The proposed chageset is located at:

https://cr.openjdk.java.net/~naoto/8233048/webrev.00/

java.time.temporal.WeekFields.ISO is instantiated using the constructor, 
which is a different object returned from WeekFields.of(MONDAY, 4). Even 
though this itself does not cause any issues, since objects are "equal", 
but the fix will provide the identity, as WeekFields objects are 
supposed to be singletons.

Naoto