fix exception on binary data

[vlada-dudr]

fixes #497

[meldafert]

While that does fix the exception (which is very helpful, thank you!), what is displayed is misleading: It is not possible to differentiate between a string containing base64 encoded data and a string containing binary data.
It would be better if the displayed type would also change to communicate that this is a string containing binary data.

[vlada-dudr]

It definitely would. I stopped using vdebug and my Python knowledge is near zero so i am not going to work on it. Feel free to pick it up.