Add missing test case.

Author: Gary Gregory

src/test/java/org/apache/commons/io/FileUtilsTestCase.java

@@ -3072,6 +3072,14 @@ public void testWriteStringToFile3() throws Exception {
         TestUtils.assertEqualContent(text, file);
     }
 
+    @Test
+    public void testWriteStringToFile4() throws Exception {
+        final File file = new File(temporaryFolder, "subdir/write.txt");
+        FileUtils.writeStringToFile(file, "Hello /u1234", (Charset) null);
+        final byte[] text = "Hello /u1234".getBytes();
+        TestUtils.assertEqualContent(text, file);
+    }
+
     @Test
     public void testWriteStringToFileWithEncoding_WithAppendOptionFalse_ShouldDeletePreviousFileLines() throws Exception {
         final File file = TestUtils.newFile(temporaryFolder, "lines.txt");