Use GenServer over :gen_server as the behaviour for GenServers

We can do this now since we support @optional_callbacks and can thus
have the GenServer.format_status/2 optional callback.

Author: whatyouhide

lib/elixir/lib/gen_server.ex

@@ -436,7 +436,7 @@ defmodule GenServer do
   @doc false
   defmacro __using__(_) do
     quote location: :keep do
-      @behaviour :gen_server
+      @behaviour GenServer
 
       @doc false
       def init(args) do

lib/elixir/test/elixir/module_test.exs

@@ -122,7 +122,7 @@ defmodule ModuleTest do
   ## Attributes
 
   test "reserved attributes" do
-    assert List.keyfind(ExUnit.Server.__info__(:attributes), :behaviour, 0) == {:behaviour, [:gen_server]}
+    assert List.keyfind(ExUnit.Server.__info__(:attributes), :behaviour, 0) == {:behaviour, [GenServer]}
   end
 
   test "registered attributes" do