prevent invalid XML node names

Format::to_xml() method can return invalid XML if the node name starts with a number or if it contains only numeric and other non-alpha characters. To fix, we remove any non-alphanumeric characters before checking to see if the key can be used to instantiate a new DOMElement.

Example of invalid results before this patch:
oil c
$array('30:0' => 'thing');
print_r(\Format::forge($array)->to_xml());