Corrected hard-coded charset to one obtained from $this->getCharset()

See following bug report: https://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31724

This is a correction of the previous update, where the charset was added, but as a hard-coded string instead of obtaining it from $this->getCharset().

Tested on Joomla 1.5, but needs testing on current branch - I'll add a test procedure to the bug report.