add headers with a fluent builder API.

```java
MockResponse response = new MockResponse()
    .addHeader("Content-Type", "application/json; charset=utf-8")
    .addHeader("Cache-Control", "no-cache")
    .setBody("{}");
```

MockResponse can be used to simulate a slow network. This is useful for
testing timeouts and interactive testing.

```java
response.throttleBody(1024, 1, TimeUnit.SECONDS);
```

import assertk.assertThat
import assertk.assertions.isCloseTo
import assertk.assertions.isFalse
import assertk.assertions.isInstanceOf
import assertk.assertions.matchesPredicate
import java.util.Deque
import java.util.concurrent.ConcurrentLinkedDeque
import java.util.concurrent.TimeUnit
import okhttp3.ConnectionEvent.NoNewExchanges
import okhttp3.internal.connection.RealConnection
import okio.IOException
import org.junit.jupiter.api.Assertions

 * limitations under the License.
 */
package okhttp3.containers

import assertk.assertThat
import assertk.assertions.contains
import java.net.InetSocketAddress
import java.net.Proxy
import java.net.Proxy.Type.SOCKS
import okhttp3.HttpUrl.Companion.toHttpUrl
import okhttp3.OkHttpClient
import okhttp3.Request
import okhttp3.containers.BasicMockServerTest.Companion.MOCKSERVER_IMAGE

 * limitations under the License.
 */
@file:Suppress("INVISIBLE_MEMBER", "INVISIBLE_REFERENCE")

package okhttp3.logging

import java.io.IOException
import java.nio.charset.Charset
import java.util.TreeSet
import java.util.concurrent.TimeUnit
import okhttp3.Headers
import okhttp3.HttpUrl
import okhttp3.Interceptor
import okhttp3.OkHttpClient
import okhttp3.Response

 */
package okhttp3

import assertk.assertThat
import assertk.assertions.isEqualTo
import assertk.assertions.isTrue
import java.io.IOException
import java.io.InputStreamReader
import java.io.Reader
import java.nio.charset.StandardCharsets
import java.util.concurrent.atomic.AtomicBoolean
import kotlin.test.assertFailsWith
import okhttp3.MediaType.Companion.toMediaType

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3

import assertk.assertThat
import assertk.assertions.containsExactly
import java.util.ArrayDeque
import java.util.Deque

class RecordingCookieJar : CookieJar {
  private val requestCookies: Deque<List<Cookie>> = ArrayDeque()
  private val responseCookies: Deque<List<Cookie>> = ArrayDeque()

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3.internal

import java.net.Authenticator
import java.net.PasswordAuthentication

class RecordingAuthenticator(
  private val authentication: PasswordAuthentication? =
    PasswordAuthentication(
      "username",
      "password".toCharArray(),
    ),

    are currently being read or written. On Windows the file system forbids this so we must be more
    careful when deleting and renaming files.

 *  Fix: Don't crash on Java 8u252 which introduces an API previously found only on Java 9 and
    above. See [Jetty's overview][jetty_8_252] of the API change and its consequences.

 *  New: `MultipartReader` is a streaming decoder for [MIME multipart (RFC 2045)][rfc_2045]

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3

import assertk.assertThat
import assertk.assertions.containsExactly
import java.net.InetAddress
import java.net.UnknownHostException
import okio.Buffer

class FakeDns : Dns {
  private val hostAddresses: MutableMap<String, List<InetAddress>> = mutableMapOf()

 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package okhttp3

import java.net.InetAddress
import java.net.UnknownHostException
import okhttp3.Dns.Companion.SYSTEM

/**
 * A domain name service that resolves IP addresses for host names. Most applications will use the