(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.