Fix Path.join/1 for lists of one element (#5639)

Also fix the spec for Path.join/1 to mention that the input list of paths has to
be non-empty, and fix the documentation to mention that this function takes a
list of paths (Path.t), not strings like it said before.

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

lib/elixir/lib/path.ex

@@ -428,10 +428,10 @@ defmodule Path do
   end
 
   @doc """
-  Joins a list of strings.
+  Joins a list of paths.
 
-  This function should be used to convert a list of strings to a path.
-  Note that any trailing slash is removed on join.
+  This function should be used to convert a list of paths to a path.
+  Note that any trailing slash is removed when joining.
 
   ## Examples
 
@@ -445,11 +445,11 @@ defmodule Path do
       "/foo/bar"
 
   """
-  @spec join([t]) :: binary
+  @spec join(nonempty_list(t)) :: binary
   def join([name1, name2 | rest]), do:
     join([join(name1, name2) | rest])
   def join([name]), do:
-    name
+    IO.chardata_to_string(name)
 
   @doc """
   Joins two paths.

lib/elixir/test/elixir/path_test.exs

@@ -211,6 +211,8 @@ defmodule PathTest do
     assert Path.join(['/foo/', "/bar/"]) == "/foo/bar"
     assert Path.join(["/", ""]) == "/"
     assert Path.join(["/", "", "bar"]) == "/bar"
+    assert Path.join(['foo', [?b, "a", ?r]]) == "foo/bar"
+    assert Path.join([[?f, 'o', "o"]]) == "foo"
   end
 
   test "join two" do