Fix duplicated keys in nested XML arrays (#525)

[z38]

At the moment nested arrays in XML configuration files are parsed incorrectly because DOMElement::getElementsByTagName not only returns immediate children but all descendants.

Example:

<array>
    <element key="a">
        <array>
            <element key="b">
                <string>foo</string>
            </element>
        </array>
    </element>
    <element key="c">
        <string>bar</string>
    </element>
</array>

array (
  'a' => array (
    'b' => 'foo',
  ),
  'b' => 'foo', // duplicated
  'c' => 'bar',
)

[sebastianbergmann]

Cherry-picked into 4.8 and merged from there. Thanks!