title: Use toString() in Mongodb projection
body: <p>I am not sure if this is a MongoDB issue I am facing or one of the tools ( NoSQLBooster , Studio 3T ) . I can use the following code in NoSQLBooster and it works. But Studio 3T complains about invalid JSON. I think I prefer studio 3T as it seems to have a lot more features , is there a workaround to make this valid JSON?  It has a problem with the .toString() parts of the projection saying "invalid JSON" ( although this will work in NoSQLBooster ) .</p>
  
  <pre><code>$project:
      {
          _id: 0,
          "Country": "$country",
          "Zip From": {
              $cond: {
                  if: { "$lt": [{ "$strLenCP": "$postcodeFrom" }, 4] },
                  then: { $concat: ["0", "$postcodeFrom".toString()] },
                  else: "$postcodeFrom".toString()
              }
          },
          "FRPUW": "0",
          "Estimated Delivery (days)": "$rate.serviceDeliveryDays"
      }
  </code></pre>