Use json.dumps() with ensure_ascii=False

[Jason Gerard DeRose]

Well, this is a bit embarrassing. Thinking that Python3 is super safe when it comes to character encodings, I haven't bothered to add any non-ascii test values yet. That was a mistake.

json.dumps() has a bad, very un-Python3-like default of ensure_ascii=True, which means you get this:

>>> json.dumps('™')
'"\\u2122"'

When you want this:

>>> json.dumps('™', ensure_ascii=False)
'"™"'

This bug is to track the fix.