feat: support array within array in 3.x

[hacksparrow]

Description

POST request body:

 { items": [
    [
      {"name": "APPLE"},
      {"serial": 67}
    ]
  ]
}

Currently stored as:

 { "items": [
    {
      "0": {
        "name": "APPLE"
      },
      "1": {
        "serial": 67
      }
    }
  ]
}

Will be stored as:

 { items": [
    [
      {
        "name": "APPLE"
      },
      {
        "serial": 67
      }
    ]
  ]}

Related issues

strongloop/loopback#4132

Checklist

• New tests added or existing tests modified to cover all changes
• Code conforms with the style
  guide

[bajtos]

Is this a back-port of #1699? Was the back-port trivial, or did you have to make any significant changes?

Let's wait until the first pull request against master is landed and then proceed with reviewing backports.

[hacksparrow]

Is this a back-port of #1699? Was the back-port trivial, or did you have to make any significant changes?

Trivial.

However, it is not so for 2.x. Still working on it.

[bajtos]

Is this a back-port of #1699? Was the back-port trivial, or did you have to make any significant changes?

Trivial.

👍