application-json no longer considered "binary" after migration from spray to akka

[rabbah]

so we can support json without base64 encoding/decoding for http web actions.
this will make a lot of users happy.

we'll support both modes for a while and then drop the requirement since it's just too common a case.

[rabbah]

To clarify the port preserved the existing behavior. We will have to change the web action handler to eschew the decoding and hence support both new and old formats.

[rabbah]

The proposed patch in #2609 allows an HTTP web action to respond with a JSON value without having to base64 encode it. As in

function main(args) { return { statusCode: 201, body: args } }

Supplying an already decoded JSON value as an argument however where it is otherwise base64 encoded today requires some care - perhaps we need a compatibility mode option. Note that this applies only for raw HTTP actions which are not the common cases. So maybe we do nothing here.