jsbox unicode stuff

We're seeing a lot of unicode-related errors being logged from jsbox apps, which seems to indicate implicit bytes<->unicode conversions somewhere.
All three things in the message are supposed to be ASCII, so I'd prefer to get exceptions if they aren't. At worst, this gets us back to our current situation but without the implicit conversion silliness. The failure mode for UTF-8 encoding is that our logs fill up with non-ASCII bytes which can potentially break whatever we feed them to.