Override the method head() in your theme, e.g. like this:
public function head() {
$this->output(
'<head>',
'<meta http-equiv="content-type" content="text/html; charset=' . $this->content['charset'] . '"/>'
);
...
$this->output('</head>');
}
However, according to MDN the two <meta> element notations are equivalent, so why even bother?