The Graph::properties() and Resource::properties() methods don't return properties in unknown namespaces

[njh]

Originally raised by @zozlak here:
https://github.com/sweetyrdf/easyrdf/issues/17

$g = new EasyRdf\Graph();
$r = $g->resource('http://sample/resource');
$r->addLiteral('dc:title', 'sample title');
$r->addLiteral('http://unknown.namespace/property', 'sample value');
print_r($r->properties());
print_r($r->propertyUris());

gives

Array(
    [0] => dc:title
)
Array(
    [0] => http://purl.org/dc/terms/title
    [1] => http://unknown.namespace/property
)

The unexpected result is caused by https://github.com/easyrdf/easyrdf/blob/master/lib/Graph.php#L1247-L1250 which silently skips all properties within namespaces unknown to EasyRdf\RdfNamespace.