[IO-324] Add Charset sister APIs to method that take a String charset name.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/io/trunk@1326612 13f79535-47bb-0310-9956-ffa450edef68

Author: garydgregory

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

@@ -1545,7 +1545,7 @@ public void testWriteStringToFile1() throws Exception {
 
     public void testWriteStringToFile2() throws Exception {
         File file = new File(getTestDirectory(), "write.txt");
-        FileUtils.writeStringToFile(file, "Hello /u1234", null);
+        FileUtils.writeStringToFile(file, "Hello /u1234", (String)null);
         byte[] text = "Hello /u1234".getBytes();
         assertEqualContent(text, file);
     }