* Don't need to worry about intermediate responses like redirects and retries.
 * Are always invoked once, even if the HTTP response is served from the cache.
 * Observe the application's original intent. Unconcerned with OkHttp-injected headers like `If-None-Match`.
 * Permitted to short-circuit and not call `Chain.proceed()`.
 * Permitted to retry and make multiple calls to `Chain.proceed()`.