multiple array append in shift is not working as expected

[JoshuaC215]

As described in #77 spec:

{
  "operation": "shift",
  "spec": {
    "arr[+]": "key1",
    "arr[+]": "key2",
    "arr[+]": "key3"
  }
}

Input:

{ 
   "key1": "field1",
   "key2": "field2",
   "key3": "field3"
}

Expected output: {"arr":["field1", "field2", "field3"]} Actual output: {"arr":["field3"]}