[CSV-203]

withNullString value is printed without quotes when QuoteMode.ALL is
specified; add QuoteMode.ALL_NON_NULL. PR apache#17.

Author: garydgregory

src/main/java/org/apache/commons/csv/CSVFormat.java

@@ -362,7 +362,8 @@ public CSVFormat getFormat() {
     		.withIgnoreEmptyLines(false)
             .withQuote(null)
             .withRecordSeparator(LF)
-            .withNullString("\\N");
+            .withNullString("\\N")
+    		.withQuoteMode(QuoteMode.ALL_NON_NULL);
     // @formatter:off
 
     /**