Check for required type field.

[wting]

Here's an example response model that's missing a type field for time_created:

  TokenResponse:
    additionalProperties: false
    properties:
      is_foo:
        type: boolean
      time_created:
        description: The creation time of the access token in ISO8601 format.
        # type: string
    required:
    - is_foo
    type: object

While the time_created field isn't required, when you use that field the Bravado client fails due to lack of the type: string field. This spec currently passes the Swagger validator.