So that single Strings can be POSTed without JSON conversion.
There's still some work to do to support single POJOs in JSON, and
to reach parity with the WebFlux reactive type handlers, but it's
now closer to what we had before we moved the String conversion
out of the function layer.
There is one behaviour change (you have to POST an array to a function
endpoint if you are sending a body), and also a message converter that
needed to be removed so that arrays are not toStringed in the response
body.
