[Bug report] Custom Data Source API + Drillthrough sends calculated fields with select request for flat values

Our backend was reporting an unknown field when we use drillthrough and there are calculated values in the report. 

We have strict checks in our Custom Data Source implementation. Replicating this against Flexmonsters implementation works because it ignores the unknown fields: https://jsfiddle.net/ezk84/y436snk8/3/

Right-clicking a cell and selecting Drillthrough while looking at the Network requests console you'll notice the POST /select has this payload:

{
  "type": "select",
  "index": "fm-product-sales",
  "query": {
    "limit": 1000,
    "filter": {
      "type": "and",
      "value": [
        {
          "field": {
            "uniqueName": "Color"
          },
          "include": [
            {
              "member": "blue"
            }
          ]
        },
        {
          "field": {
            "uniqueName": "Country"
          },
          "include": [
            {
              "member": "Australia"
            }
          ]
        }
      ]
    },
    "fields": [
      {
        "uniqueName": "Color"
      },
      {
        "uniqueName": "Country"
      },
      {
        "uniqueName": "Price"
      },
      {
        "uniqueName": "Quantity"
      },
      {
        "uniqueName": "Min Quantity"
      }
    ]
  },
  "page": 0
}

We've updated our backend to ignore unknown fields in this scenario, but this is possibly a bug in the FM front-end.