*
 * Username and password are either present, or the empty string `""` if absent. This class offers
 * no mechanism to differentiate empty from absent. Neither of these components are popular in
 * practice. Typically HTTP applications use other mechanisms for user identification and
 * authentication.
 *
 * ### Host
 *
 * The host identifies the webserver that serves the URL's resource. It is either a hostname like

 *  **Canceling batches of calls is now the application's responsibility.**
    The API to cancel calls by tag has been removed and replaced with a more
    general mechanism. The dispatcher now exposes all in-flight calls via its
    `runningCalls()` and `queuedCalls()` methods. You can write code that
    selects calls by tag, host, or whatever, and invokes `Call.cancel()` on the