'I made it so that it could parse both normal queries (eg, `?age=12&cards=visa&cards=mastercard`) and JSON (eg, `?age=12&cards=["visa","mastercard"]`).'
It's worth pointing out, since even here in 2015 people often don't realize this, that the querystring is permitted by the standard to contain more than one value of the same type. Thus, your second example in this case could also be cards=visa&cards=mastercard.
This is particularly a problem when you've got an overly-helpful web framework in a dynamically-typed language that normally exposes querystring parameters as a string, but if more than one gets passed in, suddenly you have an array. This can cause all sorts of fun problems, including security failures, though usually just bugs.
(Even in a dynamically typed language, a web framework should either represent querystring parameters as an array at all times xor enforce a clear, stated policy of which string it accepts if it is going to pick just one, but it should definitely not just split the difference. The latter, despite in some sense being "wrong", does have the advantage of matching people's mental model, which is a useful enough thing on its own.)
This is, incidentally, a useful "Are the developers at least this tall?" test to apply to your new Web Framework o' the Week.