nixpkgs-github-update: don't malform header

pkgs/desktops/pantheon/nixpkgs_github_update/lib/nixpkgs_github_update/github_latest_version.ex

@@ -3,15 +3,9 @@ defmodule NixpkgsGitHubUpdate.GitHubLatestVersion do
 
   def fetch({owner, repo}) do
     endpoint = releases_endpoint(owner, repo)
+    headers = construct_headers()
 
-    oauth_token = String.to_charlist("#{System.get_env("OAUTH_TOKEN")}")
-
-    headers = %{
-      'User-Agent' => @user_agent,
-      'Authorization' => 'token #{oauth_token}'
-    }
-
-    :httpc.request(:get, {endpoint, Map.to_list(headers)}, [], [])
+    :httpc.request(:get, {endpoint, headers}, [], [])
     |> handle_response
   end
 
@@ -19,6 +13,23 @@ defmodule NixpkgsGitHubUpdate.GitHubLatestVersion do
     'https://api.github.com/repos/#{owner}/#{repo}/releases/latest'
   end
 
+  def construct_headers do
+    headers = %{'User-Agent' => @user_agent}
+
+    put_token(headers, get_token())
+    |> Map.to_list
+  end
+
+  defp get_token do
+    String.to_charlist("#{System.get_env("OAUTH_TOKEN")}")
+  end
+
+  defp put_token(headers, token) when is_binary(token) do
+    Map.put_new(headers, 'Authorization', 'token #{token}')
+  end
+
+  defp put_token(headers, _), do: headers
+
   def handle_response({_, {{_httpv, status_code, _}, _headers, response}}) do
     {
       status_code |> check_for_error(),