Java9 DateFormat changes

[amogilev]

The default formats used in DateFormat has changes in Java 9. For example:

• Java8 (and below): "Jan 1, 1970 12:00:00 AM"
• Java9: "Jan 1, 1970, 12:00:00 AM"
• Java8 (and below): "Thursday, January 1, 1970 12:00:00 AM UTC"
• Java9: "Thursday, January 1, 1970 at 12:00:00 AM Coordinated Universal Time"

As a result, a number of Gson unit tests currently fail when run on Java 9. Additionally, this causes some sort of a backward incompatibility: a date written by Gson in Java 8 cannot be parsed back in Java 9.

In order to fix it, I suggest the following changes:

• provide a way for Gson to read default "old" formats. As Gson by default uses only US date formats, it is possible;
• change "write" unit tests so that they check the results depending on the current Java version. If run on Java 9 or later, expect the new formats to be used; otherwise, expect the "old" results