Improve doctest stripping, closes #1678

Author: José Valim

lib/ex_unit/lib/ex_unit/doc_test.ex

@@ -383,7 +383,7 @@ defmodule ExUnit.DocTest do
   end
 
   defp strip_indent(line, indent) do
-    line |> String.slice(indent, String.length(line))
+    String.slice(line, indent, String.length(line)) || ""
   end
 
   defp extract_tests([], _line, "", "", [], _) do

lib/ex_unit/test/ex_unit/doc_test_test.exs

@@ -141,7 +141,6 @@ defmodule ExUnit.DocTestTest.IndentationNotEnough do
   def not_enough, do: :ok
 end
 
-
 defmodule ExUnit.DocTestTest do
   use ExUnit.Case