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/redirect_test.go

@@ -339,11 +339,10 @@ func TestRedirectPolicy_StopsAtHopCap(t *testing.T) {
 // The set the redirect policy strips is whatever the delivery path
 // actually put on the wire, so a header added to the forward set is
 // covered without a second edit. A header the event never carried
-// is not in the set, and neither is the inbound Content-Type, because
-// it is not forwarded. Two more are deliberately excluded: a
-// Content-Type configured on the target describes the body, which a
-// 307 carries across hosts, and the inbound User-Agent every real
-// sender supplies is overwritten before the request goes out.
+// is not in the set, and the delivery path's own two are deliberately
+// excluded: Content-Type describes the body, which a 307 carries
+// across hosts, and the inbound User-Agent every real sender supplies
+// is overwritten before the request goes out.
 func TestApplyRequestHeaders_ReportsOriginScopedNames(t *testing.T) {
 	t.Parallel()
 
@@ -372,7 +371,6 @@ func TestApplyRequestHeaders_ReportsOriginScopedNames(t *testing.T) {
 		&delivery.HTTPTargetConfig{
 			Headers: map[string]string{
 				probeHeaderName: probeHeaderValue,
-				"Content-Type":  testContentType,
 			},
 		},
 	)
@@ -380,11 +378,7 @@ func TestApplyRequestHeaders_ReportsOriginScopedNames(t *testing.T) {
 	assert.Equal(t,
 		[]string{probeHeaderName, inboundHeaderName}, names,
 		"both header classes are reported, and only those: "+
-			"Host and the inbound Content-Type are never "+
-			"forwarded, User-Agent is the delivery path's own",
-	)
-	assert.NotContains(t, names, "Content-Type",
-		"a Content-Type configured on the target must survive "+
-			"a cross-origin 307/308 with the body it describes",
+			"Host is never forwarded, Content-Type and "+
+			"User-Agent are the delivery path's own",
 	)
 }