Fix bounding_box serializer

[kenkoooo]

According to the API document, the type of the coordinates field in the bounding box object is "Array of Array of Array of Float" like the following: https://developer.twitter.com/en/docs/twitter-api/premium/data-dictionary/object-model/geo

{
  "coordinates": [[
      [
        -74.026675,
        40.683935
      ],
      [
        -74.026675,
        40.877483
      ],
      [
        -73.910408,
        40.877483
      ],
      [
        -73.910408,
        40.3935
      ]
    ]]
}

We can parse it correctly, but we can not serialize it to JSON correctly. The coordinates value of a serialized JSON object will be "Array of Array of Float" like the following:

{
  "coordinates": [
      [
        -74.026675,
        40.683935
      ],
      [
        -74.026675,
        40.877483
      ],
      [
        -73.910408,
        40.877483
      ],
      [
        -73.910408,
        40.3935
      ]
    ]
}

In this PR, I fixed the serializer and added a test for it.

[adwhit]

Thanks, LGTM

[kenkoooo]

Thank you for the review!