Fix join helper for arrays of primitives

[Shaisolaris]

The join helper fails for arrays of primitives:

handlebars.registerHelper("join", StringHelpers.join);
handlebars.compileInline("{{{join this \", \"}}}").apply(new int[] {6, 7, 8});
// java.lang.ClassCastException: class [I cannot be cast to class [Ljava.lang.Object;

StringHelpers.join casts every array to Object[], which only works for arrays of objects. This change copies the elements with java.lang.reflect.Array, so int[], long[], double[] and so on are joined like String[] already is.

Tests: added StringHelpersTest.joinPrimitiveArray. It fails before the change (ClassCastException) and passes after. mvn test in the handlebars module passes locally (1039 tests, 3 skipped). spotless:check lists the same files with format violations as on master. StringHelpers.java was already in that list on master. I did not run the other modules.