Fallback from iconv to mbstring for encoding convertion

In some exotic environnement (IBM AIX) it may happens that `//TRANSLIT` is not supported. In that case iconv would return false. We can easily identify the case and fallback on mbstring instead. That should give correct result.

Closes #135
Closes #136

src/PhpSpreadsheet/Shared/StringHelper.php

@@ -468,7 +468,10 @@ class StringHelper
     public static function convertEncoding($value, $to, $from)
     {
         if (self::getIsIconvEnabled()) {
-            return iconv($from, $to . '//IGNORE//TRANSLIT', $value);
+            $result = iconv($from, $to . '//IGNORE//TRANSLIT', $value);
+            if (false !== $result) {
+                return $result;
+            }
         }
 
         return mb_convert_encoding($value, $to, $from);