CRM-12647 - CRM_Utils_Migrate - Extract constant for special delimiter

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* CRM-12647: Customization export/import crashes on "&"
  http://issues.civicrm.org/jira/browse/CRM-12647

CRM/Utils/Migrate/Export.php

@@ -34,6 +34,8 @@
  */
 class CRM_Utils_Migrate_Export {
 
+  const XML_VALUE_SEPARATOR = ":;:;:;";
+
   protected $_xml;
 
   function __construct() {
@@ -405,7 +407,7 @@ AND    entity_id    IS NULL
         }
         else {
           $value = str_replace(CRM_Core_DAO::VALUE_SEPARATOR,
-            ":;:;:;",
+            self::XML_VALUE_SEPARATOR,
             $object->$name
           );
           $xml .= "\n      <$name>$value</$name>";

CRM/Utils/Migrate/Import.php

@@ -92,7 +92,7 @@ class CRM_Utils_Migrate_Import {
     foreach ($fields as $name => $dontCare) {
       if (isset($xml->$name)) {
         $value = (string ) $xml->$name;
-        $value = str_replace(":;:;:;",
+        $value = str_replace(CRM_Utils_Migrate_Export::XML_VALUE_SEPARATOR,
           CRM_Core_DAO::VALUE_SEPARATOR,
           $value
         );