json casts

I've been on the receiving end of a couple of mumbles about the fact
that the JSON rendering code ignores casts of builtin types to JSON.
This was originally done as an optimization to avoid doing cache lookups
for casts for things we knew quite well how to turn into JSON values
(unlike, say, hstore). However, there is at least one concrete case
where this has some possibly undesirable consequences, namely
timestamps. Many JSON processors, especially JavaScript/ECMAScript
processors, require timestamp values to be in ISO 8601 format, with a
'T' between the date part and the time part, and thus they barf on the
output we produce for such values.

I'm therefore wondering if we should just remove this optimization. I
don't have any performance figures, but it seems unlikely to be terribly
expensive. Perhaps in 9.5 we should look at caching a lot of the info
the json rendering code gathers, but that's something we can't look at now.

cheers

andrew