Encode strings as JSON in `where` clauses

[dantownsend]

We just added more powerful JSON filtering, but when integrating it into Piccolo Admin, I realised there's a bug here:

piccolo/piccolo/query/operators/json.py

Lines 15 to 16 in 448a818

	if not isinstance(value, (str, QueryString)):
	value = dump_json(value)

We should also encode strings as JSON e.g. 'hello world' -> '"hello world"'. However, if the string is already valid JSON, we should leave it alone (e.g. '{"message": "hello world"}').