fixes `adopt_as' to relink the namespace of the new node

previously `adopt_as' would call `relink_namespace' on the parent
node. this caused a loop such as the following

  1000.times {|i| root << root.create_element('randomNode') }

to run in O(n^2) instead of in linear time, since relink_namespace was
relinking the root node + 1 child node initially, then the root node + 2
nodes when the second node is added, etc.

fixes #692

ext/java/nokogiri/XmlNode.java

@@ -1579,7 +1579,7 @@ protected IRubyObject adoptAs(ThreadContext context, AdoptScheme scheme,
             ((XmlDocument) this).resetNamespaceCache(context);
         }
 
-        relink_namespace(context);
+        other.relink_namespace(context);
         // post_add_child(context, this, other);
 
         return nodeOrTags;