Implement \Dxw\Assertions\XML

[mallorydxw]

API should be identical to \Dxw\Assertions\HTML.

Ideally we should convert both documents to Canonical XML.

Here's a bit of code we used on another project:

    protected function assertXMLEquals($expected, $actual)
    {
        call_user_func_array(
            [$this, 'assertEquals'],
            array_map(
                function ($x) {
                    $d = new DOMDocument;
                    $d->loadXML($x);

                    $t = $d->C14N();
                    $t = preg_replace('/^\s+/m', '', $t);
                    $t = preg_replace('/\n/', '', $t);
                    $t = preg_replace('/></', ">\n<", $t);
                    return $t;
                },
                [$expected, $actual]
            )
        );
    }