APIB: Use a default media type for requests and responses when none is specified

[chiiya]

The following Response definition in API Blueprint does not have a media type specified, but is still parsed correctly by Drafter.

+ Response 200
   + Body
        ```json
        [
            {
                "id": 2
            },
            {
                "id": 4
            }
        ]
        ```    
    + Schema
        ```json
        :[](schemas/account/responses/users.json)
        ```

The body definition is ignored by comet. A default media type (* or default) should be used in this case.
Relevant section: packages/adapter-api-blueprint/src/transformers/ResponseTransformer:39

if (mediaType !== null) {
  // This block is never entered
}