fix a backwards compatibility bug with maps

Do an unlet <old key> when over riding a method on a prototype object.
Some older versions of vim spaz out otherwise.

Note: i was using these unlets up until a few versions ago when the were
refactored out

doc/NERD_tree.txt

@@ -901,6 +901,10 @@ fridge for later ;)
 ==============================================================================
 7. Changelog                                               *NERDTreeChangelog*
 
+2.14.2
+    - compatibility bugfix for older version of vim, thanks to knekk for
+      helping me track it down
+
 2.14.1
     - dont clobber &cpo. Thanks to godlygeek for the bug report.
 
@@ -1227,6 +1231,9 @@ handling named pipes.
 
 Chur to godlygeek for reporting a bug where &cpo was getting clobbered.
 
+Cheers to knekk for helping me track down a bug when overwriting dictionary
+keys that only occurred in some versions of vim.
+
 ==============================================================================
 9. License                                                   *NERDTreeLicense*
 

plugin/NERD_tree.vim

@@ -713,6 +713,7 @@ endfunction
 "
 "Args:
 "path: a path object
+unlet s:TreeDirNode.findNode
 function! s:TreeDirNode.findNode(path)
     if a:path.equals(self.path)
         return self
@@ -731,7 +732,6 @@ function! s:TreeDirNode.findNode(path)
     endif
     return {}
 endfunction
-
 "FUNCTION: TreeDirNode.getChildCount() {{{3
 "Returns the number of children this node has
 function! s:TreeDirNode.getChildCount()
@@ -887,6 +887,7 @@ endfunction
 "
 "Args:
 "path: a path object representing the full filesystem path to the file/dir that the node represents
+unlet s:TreeDirNode.New
 function! s:TreeDirNode.New(path)
     if a:path.isDirectory != 1
         throw "NERDTree.TreeDirNode.InvalidArguments exception. A TreeDirNode object must be instantiated with a directory Path object."
@@ -948,6 +949,7 @@ function! s:TreeDirNode._openRecursively2(forceOpen)
 endfunction
 
 "FUNCTION: TreeDirNode.refresh() {{{3
+unlet s:TreeDirNode.refresh
 function! s:TreeDirNode.refresh()
     call self.path.refresh()