Feature/490 json attributes as object

### Description
The rendering of ContextAttribute for JSON has now two possibilities:
1. As a vector (the old way)
2. As an object, with each attribute as an object inside

(1) is the default rendering method, while the second method is achieved adding the URL parameter

    attributesFormat=object

Fixes issue #490.
Of course, I forgot.
You might have seen I have changed this to the '{} way' in hundreds of places already.
Missed this one, sorry (force of habit).