[interchange] Improve application/json POST support

[Jon Jensen]

On Tue, 30 Dec 2014, Peter wrote:

> You've added in JSON as a requirement here, but it's not (last time I 
> checked) part of core perl and it's not listed as a module in 
> Bundle::Interchange (or even Bundle::InterchangeKitchenSink).  Please 
> add in code that makes this optional so we don't break existing IC sites 
> when they upgrade.

Good idea -- in fact, I think that's probably why we didn't add 
auto-decoding of JSON in the beginning.

> Mike, should JSON be a part of KitchenSink?

My name's not Mike, but I say +1 to that. :)

Jon