(PUP-391) Fix Rdoc 4+ specs on Windows Ruby 2 x64

 - 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).  NOTE: This does not affect RDoc 3.9.5
   that ships with Ruby 1.9.3 on Windows, only RDoc 4.0.0+ that ships
   with Ruby 2.0.0+ on Windows.
 - 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, specifically 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.
(PUP-391) Ruby 2 exception type changed to EISDIR

 - File.open now raises EISDIR when a path is a directory, prior to
   verifying permissions in Ruby 2.  Files continue to return EACCESS.