phpmailer MDL-24876 Changed var from private to public and noted in readme file

lib/phpmailer/README_MOODLE.txt

@@ -13,4 +13,4 @@ test/
 Our changes:
  * Modified EncodeQP() in class.phpmailer.php to not use php's implementation of quoted
    printable encoding as it was causing problems for some users MDL-23240
-
+ * Changed PHPMailer::ReplyTo from private to public as it is used in moodlelib.php

lib/phpmailer/class.phpmailer.php

@@ -310,7 +310,7 @@ class PHPMailer {
   private   $to             = array();
   private   $cc             = array();
   private   $bcc            = array();
-  private   $ReplyTo        = array();
+  public   $ReplyTo        = array();
   private   $all_recipients = array();
   private   $attachment     = array();
   private   $CustomHeader   = array();