fix for Xpath.getElementTreeXPath

I came accross this issue when I reused getElementTreeXPath() functionality in my project

Case:

  Incorrect Xpath string returned if element has siblings of same node name when traversing element node tree

Example (how to reproduce):

  Using Firebug's "Copy XPath" when right clicking on element in document's element tree.

  Let's say we have we have a HTML document with this structure, where X is the element we're trying to get XPath of:

  - html
  -- body
  --- div
  ---- div [X]
  ---- div

  XPath returned: /html/body/div/div (should be /html/body/div/div[1])