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);

    val headers = headersOf("Cache-Control", "max-age=12", "Pragma", "must-revalidate", "Pragma", "public")
    val cacheControl = parse(headers)
    assertThat(cacheControl.toString()).isEqualTo("max-age=12, public, must-revalidate")
  }

  @Test
  fun parseCacheControlHeaderValueIsRetained() {
    val value = "max-age=12"
    val headers = headersOf("Cache-Control", value)
    val cacheControl = parse(headers)

  /**
   * Notification that the receiving stream flow control window has changed.
   * [WindowCounter] generally carries the client view of total and acked bytes.
   */
  fun receivingStreamWindowChanged(
    streamId: Int,
    windowCounter: WindowCounter,
    bufferSize: Long,
  )

  /**
   * Notification that the receiving connection flow control window has changed.

    when {
      name.equals("Cache-Control", ignoreCase = true) -> {
        if (headerValue != null) {
          // Multiple cache-control headers means we can't use the raw value.
          canUseHeaderValue = false
        } else {
          headerValue = value
        }
      }
      name.equals("Pragma", ignoreCase = true) -> {
        // Might specify additional cache-control params. We invalidate just in case.

timeout elapses. If we get bytes but there's nobody asking for them, we buffer them. We don't consider bytes as delivered for flow control until they're consumed by the application.

Consider an application streaming a video over http/2. Perhaps the user pauses the video and the application stops reading bytes from this stream. The buffer will fill up, and flow control prevents the server from sending more data on this stream. When the user unpauses her video the buffer drains, the read is acknowledged,...

  }

fun Request.Builder.commonCacheControl(cacheControl: CacheControl): Request.Builder {
  val value = cacheControl.toString()
  return when {
    value.isEmpty() -> removeHeader("Cache-Control")
    else -> header("Cache-Control", value)
  }
}

fun Request.Builder.commonGet(): Request.Builder = method("GET", null)

fun Request.Builder.commonHead(): Request.Builder = method("HEAD", null)

 * limitations under the License.
 */
package okhttp3.internal.concurrent

/**
 * A unit of work that can be executed one or more times.
 *
 * Recurrence
 * ----------
 *
 * Tasks control their recurrence schedule. The [runOnce] function returns -1L to signify that the
 * task should not be executed again. Otherwise it returns a delay until the next execution.
 *

 * limitations under the License.
 */
package okhttp3.internal.http2

import okhttp3.ConnectionListener
import okhttp3.internal.http2.flowcontrol.WindowCounter

/**
 * ConnectionListener that outputs CSV for flow control of client receiving streams.
 */
class Http2FlowControlConnectionListener : ConnectionListener(), FlowControlListener {
  val start = System.currentTimeMillis()

  override fun receivingStreamWindowChanged(

 * Fix: Change HTTP date formats to UTC to conform to RFC2616 section 3.3.
 * Fix: Support SPDY header blocks with trailing bytes.
 * Fix: Allow `;` as separator for `Cache-Control` header.
 * Fix: Correct bug where HTTPS POST requests were always automatically buffered.
 * Fix: Honor read timeout when parsing SPDY headers.


## Version 1.2.1

_2013-08-23_

    val headers =
      Headers.headersOf(
        "cache-control",
        "no-store",
        "Cache-Control",
        "no-cache",
      )
    val headerMap = headers.toMultimap()
    assertThat(headerMap["cache-control"]!!.size).isEqualTo(2)
    assertThat(headerMap["Cache-Control"]!!.size).isEqualTo(2)
  }