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.
More things to test (maybe in separate .test, following the idea of having small .test cases):

* Compound values (JSON object and vectors)
* Defining attributtes with same name and different IDs, to check that the render logic "colapse" attributes
* Defining attributtes with same name and types, to check that the render logic "colapse" attributes