[,] is intersection, not a union operator for field names

When separating fields with a comma (`,`) then this is interpreted as an intersection rather than a union.

Example: query test data (http://jsonpath.herokuapp.com/).
Query: `$..['price', 'color']`

**Actual result**
```
[
   {
      "price" : 19.95,
      "color" : "red"
   }
]
```
(objects that contain both `price` and `color`)

**Expected result**
```
[19.95, "red", ...]
```