Add test case, release 0.2.1

Author: sgfn

README.md

@@ -15,7 +15,7 @@ Implements:
 ```elixir
 def deps do
   [
-    {:ex_turn, "~> 0.2.0"}
+    {:ex_turn, "~> 0.2.1"}
   ]
 end
 ```

mix.exs

@@ -1,7 +1,7 @@
 defmodule ExTURN.MixProject do
   use Mix.Project
 
-  @version "0.2.0"
+  @version "0.2.1"
   @source_url "https://github.com/elixir-webrtc/ex_turn"
 
   def project do

test/ex_turn/client_test.exs

@@ -304,6 +304,9 @@ defmodule ExTURN.ClientTest do
       msg = {:socket_data, @turn_ip, @turn_port, data}
 
       assert {:ok, %Client{}} = Client.handle_message(client, msg)
+
+      client = %{client | state: :error}
+      assert {:error, :invalid_state, ^client} = Client.handle_message(client, msg)
     end
   end