*     exception by calling [HttpUrl.parse]; it returns null for invalid URLs.
     */
    open fun url(url: String): Builder = url(canonicalUrl(url).toHttpUrl())

    // Silently replace web socket URLs with HTTP URLs.
    private fun canonicalUrl(url: String) =
      when {
        url.startsWith("ws:", ignoreCase = true) -> "http:${url.substring(3)}"