fix insecure hosts crash with an `IllegalArgumentException` on Android.


## Version 4.7.0

_2020-05-17_

 *  New: `HandshakeCertificates.Builder.addInsecureHost()` makes it easy to turn off security in
    private development environments that only carry test data. Prefer this over creating an
    all-trusting `TrustManager` because only hosts on the allowlist are insecure. From
    [our DevServer sample][dev_server]:

      cbuf: CharArray,
      off: Int,
      len: Int,
    ): Int {
      if (closed) throw IOException("Stream closed")

      val finalDelegate =
        delegate ?: InputStreamReader(
          source.inputStream(),
          source.readBomAsCharset(charset),
        ).also {
          delegate = it
        }
      return finalDelegate.read(cbuf, off, len)
    }

Caching
=======

OkHttp implements an optional, off by default, Cache. OkHttp aims for RFC correct and
pragmatic caching behaviour, following common real-world browser like Firefox/Chrome and 
server behaviour when ambiguous.

# Basic Usage

```kotlin
  private val client: OkHttpClient = OkHttpClient.Builder()
      .cache(Cache(
          directory = File(application.cacheDir, "http_cache"),
          // $0.05 worth of phone storage in 2020

 * **Step down considerately**: Members of every project come and go. When somebody leaves or
   disengages from the project, they should make it known and take the proper steps to ensure that
   others can pick up where they left off.

This code is not exhaustive or complete. It serves to distill our common understanding of a
collaborative, shared environment, and goals. We expect it to be followed in spirit as much as in
the letter.

    This simple option dramatically changes the behavior of the request body and of the entire
    call.

    The `RequestBody.writeTo()` method may now retain a reference to the provided sink and
    hand it off to another thread to write to it after `writeTo` returns.

    The `EventListener` may now see requests and responses interleaved in ways not previously

import okhttp3.internal.connection.RoutePlanner.Plan
import okhttp3.internal.okHttpName

/**
 * Speculatively connects to each IP address of a target address, returning as soon as one of them
 * connects successfully. This kicks off new attempts every 250 ms until a connect succeeds.
 */
internal class FastFallbackExchangeFinder(
  override val routePlanner: RoutePlanner,
  private val taskRunner: TaskRunner,
) : ExchangeFinder {

#### Shared reader thread

We can't rely on application threads to read data from the socket. Application threads are transient: sometimes they're reading and writing and sometimes they're off doing application-layer things. But the socket is permanent, and it needs constant attention: we dispatch all incoming frames so the connection is good-to-go when the application layer needs it.

   * lifted. Such writes occur on an application-provided thread and may occur concurrently with
   * reads of the [ResponseBody]. For duplex request bodies, [writeTo] should return
   * quickly, possibly by handing off the provided request body to another thread to perform
   * writing.
   *
   * [grpc]: https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md
   */
  open fun isDuplex(): Boolean = commonIsDuplex()

  /**

      encodings[ 0x10] = encoding // Data Link Escape
      encodings[ 0x11] = encoding // Device Control 1 (oft. XON)
      encodings[ 0x12] = encoding // Device Control 2
      encodings[ 0x13] = encoding // Device Control 3 (oft. XOFF)
      encodings[ 0x14] = encoding // Device Control 4
      encodings[ 0x15] = encoding // Negative Acknowledgment
      encodings[ 0x16] = encoding // Synchronous idle

      assertThat(responseBody.readUtf8Line()).isNull()
    }
    body.awaitSuccess()
  }

  /**
   * Duplex calls that have follow-ups are weird. By the time we know there's a follow-up we've
   * already split off another thread to stream the request body. Because we permit at most one
   * exchange at a time we break the request stream out from under that writer.
   */
  @Test
  fun duplexWithRedirect() {