Ensure Application.spec/2 returns nil for unknown applications

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

Author: José Valim

lib/elixir/lib/application.ex

@@ -133,14 +133,16 @@ defmodule Application do
   @doc """
   Returns the value for `key` in `app`'s specification.
 
-  See `spec/1` for the supporte keys. If the given
+  See `spec/1` for the supported keys. If the given
   specification parameter does not exist, this function
-  will raise.
+  will raise. Returns `nil` if the application is not loaded.
   """
-  @spec spec(app, key) :: value
+  @spec spec(app, key) :: value | nil
   def spec(app, key) when key in @application_keys do
-    {:ok, value} = :application.get_key(app, key)
-    value
+    case :application.get_key(app, key) do
+      {:ok, value} -> value
+      :undefined -> nil
+    end
   end
 
   @doc """

lib/elixir/test/elixir/application_test.exs

@@ -38,6 +38,7 @@ defmodule ApplicationTest do
   test "application specification" do
     assert is_list Application.spec(:elixir)
     assert Application.spec(:unknown) == nil
+    assert Application.spec(:unknown, :description) == nil
 
     assert Application.spec(:elixir, :description) == 'elixir'
     assert_raise FunctionClauseError, fn -> Application.spec(:elixir, :unknown) end