PBT-823 fix deserialisation

[Andyck]

Problem:
if you create a measurement in c8y and retrieve it from c8y again, the measurement is packed into another JsonObject and the type is changed from float to string.
So the serialized object is no longer identical to the re-serialized object:

Expected: 
{
    "CalculatedPowerValuesPerMonth": {
      "solar": {
        "value": 5730347.0,
        "unit": "Wh"
      }
    },
    "id": "1234567",
    "time": "2020-06-10T13:27:31.696+02",
    "type": "CalculatedPowerValuesPerMonth"
  }

Actual: 
{
    "CalculatedPowerValuesPerMonth": {
      "solar": {
        "value": {
          "value": "5730347.0"
        },
        "unit": "Wh"
      }
    },
    "id": "1234567",
    "time": "2020-06-10T13:27:31.696+02",
    "type": "CalculatedPowerValuesPerMonth"
  }