Bugfix: nested within was resetting to top-level

[markevans]

Hi - thanks for this useful library.

This is a fix for things like the following:

<div class="page-1">
  <p>...</p>
  <button>Next</button>
</div>

<div class="page-2">
  <button>Next</button>
</div>

conn
|> within(".page-1", fn page ->
  |> within("p", fn p ->
    # assert something...
  end)
  # THIS BREAKS! due to finding multiple buttons because `within` resets scope to :none
  |> click_button("Next")
end)

within was resetting the scope to :none, rather than the parent scope.