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.
However, this has very little (nothing) to do with this PR, right?
It's a strange operation, and I'm guessing the second attribute will overwrite the first one.
I don't see why a functional test like this fits in this PR ...