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.
Although of course the CB support not using the same format in input an ouput, from a test specification point of view, is a bit weird have the requests in XML and the responses in JSON :)

Is intentional?