Replace an equality test with an instance method

A proper instance method was substituted for the more brittle
equality test in the "TreeDirNode.open()" method.

Note that the order of the tests was reversed to account for the
fact that the "isRoot()" method can only be called after the first
test has passed.

lib/nerdtree/tree_dir_node.vim

@@ -431,7 +431,7 @@ function! s:TreeDirNode.open(...)
 
     " Open any ancestors of this node that render within the same cascade.
     let l:parent = self.parent
-    while l:parent != b:NERDTree.root && !empty(l:parent)
+    while !empty(l:parent) && !l:parent.isRoot()
         if index(l:parent.getCascade(), self) >= 0
             let l:parent.isOpen = 1
             let l:parent = l:parent.parent