Description:
------------
When importing a DomNode that contains a custom entity in one of it's attributes, into another DomDocument, the entity is missing from the output of getAttribute().

Note:
This is working if the custom entity is already present in the original document. e.g if you add a attribute hack="⟨" into the root element of the original document, the output of getAttribute on the imported DomNode is as expected "index.php?lang=en"


Test script:
---------------
<?php
// Creating document for import
$xml = <<<'XSL'
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xsl:stylesheet  [<!ENTITY lang "lang=en">]>
<ex:bla href="index.php?&lang;" xmlns:ex="http://example.com" version="1.0"/>
XSL;
$source = \DomDocument::loadXml($xml);

// Creating original document
$xml = <<<'XSL'
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xsl:stylesheet  [<!ENTITY lang "lang=en">]>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:ex="http://example.com" version="1.0"/>
XSL;
$doc1 = \DomDocument::loadXml($xml);
$doc1->documentElement->appendChild($doc1->importNode($source->documentElement));

//echo $doc1->saveXml();  // xml is displayed correct
echo $doc1->documentElement->firstChild->getAttribute("href"); // getAttribute() is not working: "index.php?"

Expected result:
----------------
index.php?lang=en

Actual result:
--------------
index.php?