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.
Hey, I completely forgot about Convenience Operations!
They all use the same source code, so I have no reason to assume anything to go wrong, but still.

*Attributes with same name and different IDs* is already in the test. See E2-A3 in test 5.