Remove warning and improve coverage on Enum.into/2 tests

Author: josevalim

Diff for: lib/elixir/lib/enum.ex

@@ -1435,7 +1435,7 @@ defmodule Enum do
 
   defp into_map(%{} = enumerable), do: enumerable
   defp into_map(enumerable) when is_list(enumerable), do: :maps.from_list(enumerable)
-  defp into_map(enumerable), do: enumerable |> Enum.to_list() |> :maps.from_list(enumerable)
+  defp into_map(enumerable), do: enumerable |> Enum.to_list() |> :maps.from_list()
 
   defp into_map(%{} = enumerable, collectable),
     do: Map.merge(collectable, enumerable)

Diff for: lib/elixir/test/elixir/enum_test.exs

@@ -396,6 +396,8 @@ defmodule EnumTest do
   test "into/2" do
     assert Enum.into([a: 1, b: 2], %{}) == %{a: 1, b: 2}
     assert Enum.into([a: 1, b: 2], %{c: 3}) == %{a: 1, b: 2, c: 3}
+    assert Enum.into(MapSet.new([a: 1, b: 2]), %{}) == %{a: 1, b: 2}
+    assert Enum.into(MapSet.new([a: 1, b: 2]), %{c: 3}) == %{a: 1, b: 2, c: 3}
     assert Enum.into(%{a: 1, b: 2}, []) == [a: 1, b: 2]
     assert Enum.into(1..3, []) == [1, 2, 3]
     assert Enum.into(["H", "i"], "") == "Hi"