Send Content-Type once on a delivery (closes #246)

A delivery set Content-Type from the event's ContentType and then
added the inbound Content-Type from the event's stored headers, so
the target could receive two values. The inbound Content-Type is no
longer forwarded from the stored headers; the receiver already saved
it as the event's ContentType.

Which value wins is now stated at applyRequestHeaders: a Content-Type
configured on the target, otherwise the event's ContentType,
otherwise none.

Model: opus-5-5

internal/delivery/target_headers_test.go

@@ -11,10 +11,11 @@ import (
 	"sneak.berlin/go/webhooker/internal/delivery"
 )
 
-// Literals these tests repeat, named so that the header name and the
+// Literals these tests repeat, named so that the header names and the
 // keep-forever archive config each have one definition.
 const (
 	headerAuthorization = "Authorization"
+	headerContentType   = "Content-Type"
 	bearerValue         = "Bearer abc"
 	archiveConfigNever  = "{\"expiry\":\"never\"}"
 )