Allow json output to not require "top_child_object"

Currently, the default JSON output looks like this:

{
  "nodes" : [
    {
      "node" : {
        "nid" : 1,
        "title" : "title",
        "field_1" : "value"
      }
    }
  ]
}

In my opinion, there's an extra, unnecessary wrapper object there (the one that wraps the "node"). AFAICT, that object will always only contain one property, the "node". However, when you update the settings for "Top-level child object" and set it to a blank value, your output looks like:

{
  "nodes" : [
    {
      "" : {
        "nid" : 1,
        "title" : "title",
        "field_1" : "value"
      }
    }
  ]
}

The following is ideal, more compact and doesn't lose any of the usability of the above.

{
  "nodes" : [
    {
      "nid" : 1,
      "title" : "title",
      "field_1" : "value"
    }
  ]
}