Fix path normalization of "/.."

Closes #2404.

Author: ericmj

lib/elixir/lib/path.ex

@@ -548,6 +548,10 @@ defmodule Path do
 
   defp normalize(path), do: normalize(split(path), [])
 
+  defp normalize([".."|t], ["/"|_] = acc) do
+    normalize t, acc
+  end
+
   defp normalize([".."|t], [_|acc]) do
     normalize t, acc
   end

lib/elixir/test/elixir/path_test.exs

@@ -111,6 +111,8 @@ defmodule PathTest do
             strip_drive_letter_if_windows) == "/bar"
 
     assert Path.expand("bar/../bar", "foo") == Path.expand("foo/bar")
+
+    assert (Path.expand("/..") |> strip_drive_letter_if_windows) == "/"
   end
 
   test :relative_to do