XMLNode: var name must be always nil when the node is document.

Although Libxml2's `struct _xmlDoc` has a member named `name` that indicates name/filename/URI of the document, `var name` of `XMLDocument` on Darwin returns always nil. This commit makes the behavior the same with Darwin.

Author: YOCKOW

CoreFoundation/Parsing.subproj/CFXMLInterface.c

@@ -495,6 +495,7 @@ static inline xmlChar* _getQName(xmlNodePtr node) {
     const xmlChar* ncname = node->name;
 
     switch (node->type) {
+        case XML_DOCUMENT_NODE:
         case XML_NOTATION_NODE:
         case XML_DTD_NODE:
         case XML_ELEMENT_DECL:

Foundation/XMLNode.swift

@@ -328,9 +328,13 @@ open class XMLNode: NSObject, NSCopying {
             return returned == nil ? nil : unsafeBitCast(returned!, to: NSString.self) as String
         }
         set {
-            if case .namespace = kind {
+            switch kind {
+            case .document:
+                // As with Darwin, ignore the name when the node is document.
+                break
+            case .namespace:
                 _CFXMLNamespaceSetPrefix(_xmlNode, newValue, Int64(newValue?.utf8.count ?? 0))
-            } else {
+            default:
                 if let newName = newValue {
                     _CFXMLNodeSetName(_xmlNode, newName)
                 } else {