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.
I would have preferred to pass the selection of params that the render needs to work (in this case Format and attributeFormat) instead of all the parameters (a kind of "global variable for everything"). However, we have discussed this long and deep, so if you have preferred doing that way, it is also ok with me ;)