Andrew Dunstan: Array of Key Value pairs to Json

As I'm working on making the Redis Foreign Data Wrapper actually do something useful, it's occurred to me that one of the Json functions we're missing is the ability to turn key value arrays into Json, as we can with hstore. There's a bit of a mismatch in that  Json's treatment of numbers and booleans involves distinguishing these from strings, but it would be possible to have a "use heuristics" option as I recently did with hstore to Json conversion.

When I get done with the Foreign Data Wrapper I will probably whip up a quick extension to add this functionality.