JSON content

All,

The spec says "When an XForms Processor receives a JSON value" and points
to RFC 4627, which establishes the application/json media type.

In practice we have seen that, like for XML, there are some other media
types in use for JSON, including using the `+json` suffix. In fact, many
`+json` media types are registered at IANA [1].

We might want to clarify what "receiv[ing] a JSON value" means more
precisely (or leave it open).

-Erik

[1] http://www.iana.org/assignments/media-types/media-types.xhtml