Fix missing value for first item of enums with trailing comma

The value of the first item of an enum with a trailing comma after its
last item was not correctly initialized to 0 any more after the changes
of 74adaa5738 (see #1515) because "_last"
attribute was not set correctly in this case.

Do set it for the last item when it's followed by a comma too and add
more unit tests checking for this.

Closes #1566.

Examples/test-suite/java/java_enums_runme.java

@@ -13,6 +13,12 @@ public class java_enums_runme implements stuff {
 
   public static void main(String argv[]) 
   {
+      if (WithTrailingComma.Second != 1)
+          throw new RuntimeException("Incorrect value for Second");
+
+      if (WithTrailingCommaAndIgnoredFirstItem.SecondNonIgnoredOne != 2)
+          throw new RuntimeException("Incorrect value for SecondNonIgnoredOne");
+
       int number = 200;
 
       // Switch statement will only compile if these enums are initialised