fix #316 wrong charset for campaign-zone-zones.php etc

If apache (AddDefaultCharset-option) or nginx (charset-option) is configured to a different charset than $phpAds_CharSet, then the non-ascii chars in zonenames as displayed incorrectly when viewing e.g. "/www/admin/campaign-zone.php?clientid=XXX&campaignid=YYY".

It's fixed by just adding:
```php
// Send header with charset info
header ("Content-Type: text/html".(isset($phpAds_CharSet) && $phpAds_CharSet != "" ? "; charset=".$phpAds_CharSet : ""));
```

Similar to what is in admin-search.php

This fix also needs to be applied to "campaign-zone-link.php" and other places which get data through AJAX.