Solve the ambiguity with structured attribute values

[fgalan]

(Originally included as comment in #947)

Finally, it is not part of PR #958 solving the ambiguenty problems for compound attributes e.g.:

What I mean is that for example the following rendering for attribute A1:

"A1": {
  "value": "M1", 
  "type": "a4", 
  "m2": {
    "type": "tm2", 
    "value": "M2"
   }, 
   "m3": 3,
}

Could be interpreted in two different ways:

• As an attribute without metadata and type and which value is the following JSON object:

{
  "value": "M1", 
  "type": "a4", 
  "m2": {
    "type": "tm2", 
    "value": "M2"
   }, 
   "m3": 3,
}

• As an attribute of type "a4" and value "M1" with 2 metadata (one named "m2" with type "tm2" and value "M2" and the other named "m3" of type "number" and value 3.

[kzangeli]

Perhaps we should treat the two cases the exact same way?
I.e. if a user creates an attribute with a compound value and it coincides with 'standard stuff', why not 'assume' it is just 'standard stuff' ?

[fgalan]

With the new JSON format being worked at #1259, this ambiguity is no longer possible.