Remove redundant code path and test.

The HTTP server will not include headers with nil values even if they
are set as part of the request.

For more info, see elixir-plug/plug#220

lib/plug_require_header.ex

@@ -62,7 +62,6 @@ defmodule PlugRequireHeader do
 
   defp extract_header_key(conn, {connection_key, header_key}, callback) do
     case List.keyfind(conn.req_headers, header_key, 0) do
-      {^header_key, nil} -> callback.(conn, header_key)
       {^header_key, value} -> assign_connection_key(conn, connection_key, value)
       _ -> callback.(conn, header_key)
     end

test/plug_require_header_test.exs

@@ -19,14 +19,6 @@ defmodule PlugRequireHeaderTest do
     assert response.resp_body == ""
   end
 
-  test "block request with the required header set to nil" do
-    connection = conn(:get, "/") |> put_nil_header("x-api-key")
-    response = TestApp.call(connection, [])
-
-    assert response.status == Status.code(:forbidden)
-    assert response.resp_body == ""
-  end
-
   test "extract the required header and assign it to the connection" do
     connection = conn(:get, "/") |> put_req_header("x-api-key", "12345")
     response = TestApp.call(connection, [])
@@ -71,8 +63,4 @@ defmodule PlugRequireHeaderTest do
     assert response.status == Status.code(:bad_request)
     assert response.resp_body == "Missing header: x-secret"
   end
-
-  defp put_nil_header(%Plug.Conn{req_headers: headers} = conn, key) when is_binary(key) do
-    %{conn | req_headers: :lists.keystore(key, 1, headers, {key, nil})}
-  end
 end