Provide a way to hint property type when posting json

When posting JSON stream through http-bindings for creating nodes it
fails with the exception that the node is missing the
jcr:primartyType even if posted.

This is because the Oak servlet treat it as String instead of
name. We have similar problems (not the exception) when trying to
create a Date.

It would be nice if it would be possible to hint a property type with
something like:

{ "jcr:primartyType" : "{Name}oak:Unstructured" }