Should defaultContentType have a default?

[defaultContentType](https://github.com/asyncapi/asyncapi/blob/master/versions/2.0.0/asyncapi.md#default-content-type) doesn't have a default value and it's not listed on the AsyncAPI Object, even though it's at root level and must be there.

Should we make this field mandatory or have a default value of, for example, `application/json`?

Thanks, @jstoiko for [your work on the TCK](https://github.com/asyncapi/tck/pull/1), it helped raise the issue.