* Clarify vnd.pgrst.object+json is also Content-Type
The previous docs explained how the `Accept` header could be set to
`application/vnd.pgrst.object+json`. This change indicates the resulting
response `Content-Type` begins likewise.
While robust clients will correctly identify that
`application/vnd.pgrst.object+json` data are json, many others which
naively check the `Content-Type` is `application/json` will fail to
recognize what should be a valid `Content-Type`.
