(PUP-391) Fix Rdoc specs on Windows Ruby 2

 - When calling the document method on RDoc and the root option has not
   been specified, RDoc will provide a default in the options class:
   @root = Pathname(Dir.pwd)
 - The win32-dir monkey patches the Dir.pwd method, and instead of
   using / separators like the rest of Ruby, it uses \ as a path
   separator.
 - RDoc uses Pathname, specificially the relative_path_from method
   which expects / separators, and gets confused with \.  Therefore,
   provide a workaround to ensure that Windows uses the appropriate
   / in paths.