JSON decoder date-converter is overeager

The json decoder tries to recognise dates & reformats them so that Date/Time objects will get created. I'm pretty sure the regular expression for spotting the date is missing a couple of brackets. It has 2 sub-expressions, separated by an '|', which binds less tightly than the anchor points at either end, so the first sub-expression is only anchored at the beginning, and the second sub-expression is only anchored at the end.

The attached patch adds tests & fixes the regex.