A delivery set Content-Type from the event's ContentType and then
added the inbound Content-Type from the event's stored headers, so a
target could receive two values. The inbound Content-Type is no
longer forwarded from the stored headers; the receiver already saves
it as the event's ContentType.
Which value is sent is now stated at applyRequestHeaders: a
Content-Type configured on the target, otherwise the event's
ContentType, otherwise none. A configured one still survives a
cross-origin 307/308 with its body.
Model: opus-5-5