Set DOMAttr::$value without expanding entities

The manual refers to the DOM Level 3 Core spec which says:

"On setting, this creates a Text node with the unparsed contents of the
string. I.e. any characters that an XML processor would recognize as
markup are instead treated as literal text."

PHP is expanding entities when DOMAttr::value is set, which is
non-compliant and is a difference in behaviour compared to browser DOM
implementations.

So, when value is set, remove all children of the attribute node. Then
create a single text node and insert that as the only child of the
attribute.

Add tests.

ext/dom/attr.c

@@ -136,6 +136,7 @@ int dom_attr_value_write(dom_object *obj, zval *newval)
 {
 	zend_string *str;
 	xmlAttrPtr attrp = (xmlAttrPtr) dom_object_get_node(obj);
+	xmlNodePtr node, next;
 
 	if (attrp == NULL) {
 		php_dom_throw_error(INVALID_STATE_ERR, 1);
@@ -149,9 +150,17 @@ int dom_attr_value_write(dom_object *obj, zval *newval)
 
 	if (attrp->children) {
 		node_list_unlink(attrp->children);
+		node = attrp->children;
+		while (node) {
+			next = node->next;
+			xmlUnlinkNode(node);
+			xmlFreeNode(node);
+			node = next;
+		}
 	}
 
-	xmlNodeSetContentLen((xmlNodePtr) attrp, (xmlChar *) ZSTR_VAL(str), ZSTR_LEN(str) + 1);
+	node = xmlNewTextLen((xmlChar *) ZSTR_VAL(str), ZSTR_LEN(str));
+	xmlAddChild((xmlNodePtr) attrp, node);
 
 	zend_string_release_ex(str, 0);
 	return SUCCESS;

ext/dom/tests/DOMAttr_entity_expansion.phpt

@@ -0,0 +1,30 @@
+--TEST--
+DOMAttr entity expansion
+--FILE--
+<?php
+$doc = new DOMDocument;
+$elt = $doc->createElement('elt');
+$doc->appendChild($elt);
+$elt->setAttribute('a','&');
+print $doc->saveXML($elt) . "\n";
+
+$attr = $elt->getAttributeNode('a');
+$attr->value = '&amp;';
+print $doc->saveXML($elt) . "\n";
+
+$attr->removeChild($attr->firstChild);
+print $doc->saveXML($elt) . "\n";
+
+$elt->setAttributeNS('http://www.w3.org/2000/svg', 'svg:id','&amp;');
+print $doc->saveXML($elt) . "\n";
+
+$attr = $elt->getAttributeNodeNS('http://www.w3.org/2000/svg', 'id');
+$attr->value = '&lt;&amp;';
+print $doc->saveXML($elt) . "\n";
+
+--EXPECT--
+<elt a="&amp;"/>
+<elt a="&amp;amp;"/>
+<elt a=""/>
+<elt xmlns:svg="http://www.w3.org/2000/svg" a="" svg:id="&amp;amp;"/>
+<elt xmlns:svg="http://www.w3.org/2000/svg" a="" svg:id="&amp;lt;&amp;amp;"/>