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.
Yes, it is intentional.
I prefer XML, but the output must be in JSON for the test to work.
So, input is XML and output is JSON.

NTC