Review decision to require `RootJsonFormat` to marshal from/to Json

**Issue by [jrudolph](https://github.com/jrudolph)**
_Thursday Jun 18, 2015 at 15:02 GMT_
_Originally opened as https://github.com/akka/akka/issues/17775_

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The latest [JSON RFC 7159](https://tools.ietf.org/html/rfc7159) is less strict than other specifications before:

> A JSON text is a serialized value. Note that certain previous
> specifications of JSON constrained a JSON text to be an object or an
> array. Implementations that generate only objects or arrays where a
> JSON text is called for will be interoperable in the sense that all
> implementations will accept these as conforming JSON texts.

In spray-json the former restriction was modelled by `RootJsonFormat` which only consumes or produces json arrays or objects. Removing the restriction would make using spray-json with akka-http simpler with the potential disadvantage that we encourage a format that some peers possibly cannot understand.

On the other hand, changing to requiring `JsonFormat` would mean that suddenly Marshallers to/from most primitive types would be in scope that are likely to interfere with other ones.

/cc @sirthias

No files, tests, or entry points are named. Start by reviewing the current akka-http JSON marshalling integration and the discussion around RootJsonFormat versus JsonFormat, then compare the options with RFC 7159. Done means reaching and documenting a decision about the required format and its compatibility trade-offs.

Written by the indexing model from the issue text.