set content-type to application/activity+json for /actor

The /actor endpoint has a content-type of application/json while Pleroma expects application/activity+json according to https://git.pleroma.social/pleroma/pleroma/-/blob/develop/lib/pleroma/object/fetcher.ex#L226.

This MR sets the content-type of /actor to application/activity+json to "conform" with Pleroma, but I am not certain which is the correct content-type to set, or if there is one.