Apitools middleware to provide a callback endpoint for github oauth

apitools_github_oauth.lua

return function(request, next_middleware)
   local CLIENT_ID     = "foo"
   local CLIENT_SECRET = "bar"
   local APP_URL       = "http://localhost:9000/#overview"

   if request.uri == '/callback' then
      local session_code = request.args["code"]
      local url          = "https://github.com/login/oauth/access_token"
      local request_body = {
	 client_id     = CLIENT_ID,
	 client_secret = CLIENT_SECRET,
	 code          = session_code
      }
      local request_args = {
	 method  = "POST",
	 url     = url,
	 headers = {
	    ["User-Agent"] = "Brainslug-App",
	    Accept         = "application/json"
	 }
      }
      local result_body, result_status, result_headers = http.simple(request_args, request_body)

      -- extract the token and granted scopes
      local access_token = json.decode(result_body)['access_token']

      local redirect_location = APP_URL .. "?access_token=" .. access_token
      local redirect_code     = 302

      return {
	 status  = redirect_code,
	 headers = {
	    Location = redirect_location
	 },
	 body = ""
      }
   else
      return next_middleware()
   end
end