refactor media api

lib/weixin_authorize/api/media.rb

@@ -15,7 +15,7 @@ module WeixinAuthorize
 
       # 目前仅仅把下载链接返回给第三方开发者，由第三方开发者处理下载
       def download_media_url(media_id)
-        download_media_url = file_endpoint + "#{media_base_url}/get"
+        download_media_url = WeixinAuthorize.endpoint_url("file", "#{media_base_url}/get")
         params = URI.encode_www_form("access_token" => get_access_token,
                                      "media_id"     => media_id)
         download_media_url += "?#{params}"