iconv rather than mbstring for convert encoding

Author: Mark Baker

Classes/PHPExcel/Shared/String.php

@@ -491,14 +491,14 @@ public static function UTF8toBIFF8UnicodeLong($value)
 	 */
 	public static function ConvertEncoding($value, $to, $from)
 	{
-		if (self::getIsMbstringEnabled()) {
-			return mb_convert_encoding($value, $to, $from);
-		}
-
 		if (self::getIsIconvEnabled()) {
 			return iconv($from, $to, $value);
 		}
 
+		if (self::getIsMbstringEnabled()) {
+			return mb_convert_encoding($value, $to, $from);
+		}
+
 		if($from == 'UTF-16LE'){
 			return self::utf16_decode($value, false);
 		}else if($from == 'UTF-16BE'){