Add checkstyle rule for StringBuilder.append(char)

And replace all instances where String.Builder.append() is called with a
String of length one with append(char).

Also adds StringUtils.toStringBuilder(Collection, String).

smack-extensions/src/main/java/org/jivesoftware/smackx/xdata/Form.java

@@ -376,7 +376,7 @@ public class Form {
             sb.append(it.next());
             // If this is not the last instruction then append a newline
             if (it.hasNext()) {
-                sb.append("\n");
+                sb.append('\n');
             }
         }
         return sb.toString();