feat: AddHeader function that appends the headers list

hijack.go

@@ -378,6 +378,18 @@ func (ctx *HijackResponse) SetHeader(pairs ...string) *HijackResponse {
 	return ctx
 }
 
+// Append key-value pairs to the end of the response headers.
+// Duplicate keys will be preserved.
+func (ctx *HijackResponse) AddHeader(pairs ...string) *HijackResponse {
+	for i := 0; i < len(pairs); i += 2 {
+		ctx.payload.ResponseHeaders = append(ctx.payload.ResponseHeaders, &proto.FetchHeaderEntry{
+			Name:  pairs[i],
+			Value: pairs[i+1],
+		})
+	}
+	return ctx
+}
+
 // SetBody of the payload, if obj is []byte or string, raw body will be used, else it will be encoded as json.
 func (ctx *HijackResponse) SetBody(obj interface{}) *HijackResponse {
 	switch body := obj.(type) {

hijack_test.go

@@ -75,6 +75,8 @@ func TestHijack(t *testing.T) {
 		g.Has(ctx.Response.Headers().Get("Content-Type"), "text/html; charset=utf-8")
 
 		// override response header
+		ctx.Response.AddHeader("Set-Cookie", "key=val1")
+		// This should override the previous one
 		ctx.Response.SetHeader("Set-Cookie", "key=val")
 
 		// override response body