Let progressbar accept string for value again

Since f7614706abfdbc653de53fbc31361f9aacab8821 progressbar doesn't accept a string as argument to the `value` method. It still does for `max`.
It took me quite a while to figure out why it didn't progress, and I be I'm not the only one being caught by this, since it's very common to get the value from a json response.