Inherit XMLNS during encoding like we did in v3

[veewee]

Q	A
Type	bug
BC Break	no
Fixed issues	#87

Summary

Fixes #87

This PR makes sure encoding / decoding inherits default XMLNS declarations, just like v3 did.

$data = xml_decode(
    $xml = <<<XML
    <foo xmlns="uri://bar">
        <x:a xmlns:x="uri://x">
            <x:b xmlns="uri://aa">
                <bar>bar</bar>
            </x:b>
        </x:a>
    </foo>
    XML
);

echo $xml;
var_dump($data);
echo xml_encode($data, utf8(), pretty_print());

<foo xmlns="uri://bar">
    <x:a xmlns:x="uri://x">
        <x:b xmlns="uri://aa">
            <bar>bar</bar>
        </x:b>
    </x:a>
</foo>

array(1) {
  ["foo"]=>
  array(2) {
    ["@namespaces"]=>
    array(1) {
      [""]=>
      string(9) "uri://bar"
    }
    ["x:a"]=>
    array(2) {
      ["@namespaces"]=>
      array(1) {
        ["x"]=>
        string(7) "uri://x"
      }
      ["x:b"]=>
      array(2) {
        ["@namespaces"]=>
        array(1) {
          [""]=>
          string(8) "uri://aa"
        }
        ["bar"]=>
        string(3) "bar"
      }
    }
  }
}

<?xml version="1.0" encoding="UTF-8"?>
<foo xmlns="uri://bar">
  <x:a xmlns:x="uri://x">
    <x:b xmlns="uri://aa">
      <bar>bar</bar>
    </x:b>
  </x:a>
</foo>