Improve record and printer Coverage (apache#66)

* Improve CSVRecord and CSVPrinter coverage

* remove useless test code and test throws

* add space

Author: dota17

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

@@ -1538,6 +1538,28 @@ public void testTrimOnTwoColumns() throws IOException {
         }
     }
 
+    @Test
+    public void testNotFlushable() throws IOException {
+        final Appendable out = new StringBuilder();
+        try (final CSVPrinter printer = new CSVPrinter(out, CSVFormat.DEFAULT)) {
+            printer.printRecord("a", "b", "c");
+            assertEquals("a,b,c" + recordSeparator, out.toString());
+            printer.flush();
+        }
+    }
+
+    @Test
+    public void testCRComment() throws IOException {
+        final StringWriter sw = new StringWriter();
+        final Object value = "abc";
+        try (final CSVPrinter printer = new CSVPrinter(sw, CSVFormat.DEFAULT.withCommentMarker('#'))) {
+            printer.print(value);
+            printer.printComment("This is a comment\r\non multiple lines\rthis is next comment\r");
+            assertEquals("abc" + recordSeparator + "# This is a comment" + recordSeparator + "# on multiple lines"
+                        + recordSeparator + "# this is next comment" + recordSeparator + "# " + recordSeparator, sw.toString());
+        }
+    }
+
     private String[] toFirstRecordValues(final String expected, final CSVFormat format) throws IOException {
         return CSVParser.parse(expected, format).getRecords().get(0).values();
     }

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

@@ -301,4 +301,12 @@ public void testGetWithEnum() {
         assertEquals(recordWithHeader.get("second"), recordWithHeader.get(EnumHeader.SECOND));
         assertThrows(IllegalArgumentException.class, () -> recordWithHeader.get(EnumFixture.UNKNOWN_COLUMN));
     }
+
+    @Test
+    public void testCSVRecordNULLValues() throws IOException {
+        final CSVParser parser = CSVParser.parse("A,B\r\nONE,TWO", CSVFormat.DEFAULT.withHeader());
+        final CSVRecord csvRecord = new CSVRecord(parser, null, null, 0L, 0L);
+        assertEquals(0, csvRecord.size());
+        assertThrows(IllegalArgumentException.class, () -> csvRecord.get("B"));
+    }
 }