feat: runtime.Marshaler can support distinct content type for strea…

…med responses

runtime/handler.go

@@ -64,7 +64,13 @@ func ForwardResponseStream(ctx context.Context, mux *ServeMux, marshaler Marshal
 		}
 
 		if !wroteHeader {
-			w.Header().Set("Content-Type", marshaler.ContentType(respRw))
+			var contentType string
+			if sct, ok := marshaler.(StreamContentType); ok {
+				contentType = sct.StreamContentType(respRw)
+			} else {
+				contentType = marshaler.ContentType(respRw)
+			}
+			w.Header().Set("Content-Type", contentType)
 		}
 
 		var buf []byte

runtime/marshaler.go

@@ -48,3 +48,11 @@ type Delimited interface {
 	// Delimiter returns the record separator for the stream.
 	Delimiter() []byte
 }
+
+// StreamContentType defines the streaming content type.
+type StreamContentType interface {
+	// StreamContentType returns the content type for a stream. This shares the
+	// same behaviour as for `Marshaler.ContentType`, but is called, if present,
+	// in the case of a streamed response.
+	StreamContentType(v interface{}) string
+}