[CSV-236] Allow duplicate headers in CSV File.

Add test.

src/test/java/org/apache/commons/csv/CSVFormatTest.java

@@ -585,6 +585,11 @@ public class CSVFormatTest {
 
     }
 
+    @Test
+    public void testJiraCsv236() {
+        CSVFormat.DEFAULT.withAllowDuplicateHeaderNames().withHeader("CC","VV","VV");
+    }
+
     @Test
     public void testNewFormat() {