editMessageCaption unhandled return type

[pew-pew]

Please answer these questions before submitting your issue. Thanks!

1. What version of pyTelegramBotAPI are you using? 2.1.5
2. What OS are you using? windows 10 x64
3. What version of python are you using? 3.4

editMessageCaption method may return message (ok) or 'true' which is not handled and throws exception ValueError("json_type should be a json dict or string.")

(same with editMessageReplyMarkup)