Apply locale fix for date type adapter test.

Fixes issue 381.
diff --git a/gson/src/test/java/com/google/gson/DefaultDateTypeAdapterTest.java b/gson/src/test/java/com/google/gson/DefaultDateTypeAdapterTest.java
index 1b94f3d..966a8c1 100644
--- a/gson/src/test/java/com/google/gson/DefaultDateTypeAdapterTest.java
+++ b/gson/src/test/java/com/google/gson/DefaultDateTypeAdapterTest.java
@@ -127,7 +127,7 @@
   public void testDateSerialization() throws Exception {
     int dateStyle = DateFormat.LONG;
     DefaultDateTypeAdapter dateTypeAdapter = new DefaultDateTypeAdapter(dateStyle);
-    DateFormat formatter = DateFormat.getDateInstance(dateStyle);
+    DateFormat formatter = DateFormat.getDateInstance(dateStyle, Locale.US);
     Date currentDate = new Date();
 
     String dateString = dateTypeAdapter.serialize(currentDate, Date.class, null).getAsString();