return the namespace even if it's not in the cache

This fixes Nokogiri::XML::Attr#namespace not working when used with XmlReader,
because namespaces never get cached in that scenario.

We can't add it to the cache at this point, because the node that defined the
namespace is not available.

ext/java/nokogiri/XmlNode.java

@@ -1082,12 +1082,23 @@ public IRubyObject key_p(ThreadContext context, IRubyObject rbkey) {
 
     @JRubyMethod
     public IRubyObject namespace(ThreadContext context) {
-        if (doc instanceof HtmlDocument) return context.getRuntime().getNil();
+        Ruby runtime = context.getRuntime();
+        if (doc instanceof HtmlDocument) return runtime.getNil();
         NokogiriNamespaceCache nsCache = NokogiriHelpers.getNamespaceCacheFormNode(node);
         String prefix = node.getPrefix();
-        XmlNamespace namespace = nsCache.get(prefix == null ? "" : prefix, node.getNamespaceURI());
+        String namespaceURI = node.getNamespaceURI();
+        if (namespaceURI == null || namespaceURI == "") {
+            return runtime.getNil();
+        }
+
+        XmlNamespace namespace = nsCache.get(prefix == null ? "" : prefix, namespaceURI);
         if (namespace == null || namespace.isEmpty()) {
-            return context.getRuntime().getNil();
+          // if it's not in the cache, create an unowned, uncached namespace and
+          // return that. XmlReader can't insert namespaces into the cache, so
+          // this is necessary for XmlReader to work correctly.
+          namespace =
+            (XmlNamespace) NokogiriService.XML_NAMESPACE_ALLOCATOR.allocate(runtime, getNokogiriClass(runtime, "Nokogiri::XML::Namespace"));
+          namespace.init(null, RubyString.newString(runtime, prefix), RubyString.newString(runtime, namespaceURI), doc);
         }
 
         return namespace;