java: fix wrong variant format

Fix the fourth report of mutoh#5.

Patch by Hleb Valoshka. Thanks!!!

Author: kou

test/test_driver_jruby.rb

@@ -12,7 +12,7 @@ def setup
     end
 
     def set_locale(tag)
-      java.util.Locale.setDefault(java.util.Locale.new(tag.language, tag.region, tag.variants.to_s))
+      java.util.Locale.setDefault(java.util.Locale.new(tag.language, tag.region, tag.variants.join("_")))
     end
 
     def test_charset