Use JSON instead of #inspect to include cask container in API

[Rylan12]

This PR updates the cask JSON API's container entry to be actual JSON instead of just a ruby #inspect string.

Before:

{
  "container": "{:type=>:naked}"
}

After:

{
  "container": {
    "type": "naked"
  }
}

---

This is technically a breaking API change, but I can't imagine anyone is using the current system since it's in this weird format. If we get a complaint, we can revert. I'm marking this as critical since it is blocking the next step in the API project (loading casks directly from the API).

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Makes sense to me!