How to Mock an Http Server So It Returns an HTTPS Response in Golang

"I am trying to write a test for an HTTPS client. To do that, I mock the server behaviour as follows:

ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        resp := `{ here some JSON }`

        w.Write([]byte(resp))

}))

However, this approach does not work because I have an HTTPS client, and I get the following error: "server gave HTTP response to HTTPS client"

How can I make the server return HTTPS response?