Support `included` in create/updates

[ttrmw]

The resource parser currently ensure!s that data is the only top level key:

jsonapi-parser/lib/jsonapi/parser/resource.rb

Line 14 in 1ab3d35

Document.ensure!(document.keys == ['data'].freeze &&

but the spec does not require this. There also appears to be a bunch of discussion around sideposting and the use of included to define resources for sideposting, eg json-api/json-api#1197, so I guess this will be required eventually.

[beauby]

This has been on my radar for a while, but I'll wait until sideposting is fully spec'ed before changing the lib.