fun run() {
        val file = File("README.md")

        val request = Request.Builder()
            .url("https://api.github.com/markdown/raw")
            .post(file.asRequestBody(MEDIA_TYPE_MARKDOWN))
            .build()

        client.newCall(request).execute().use { response ->
          if (!response.isSuccessful) throw IOException("Unexpected code $response")

| RequestBody.create(ByteArray)       | ByteArray.toRequestBody()       |
| RequestBody.create(ByteString)      | ByteString.toRequestBody()      |
| RequestBody.create(File)            | File.asRequestBody()            |
| RequestBody.create(String)          | String.toRequestBody()          |
| ResponseBody.create(BufferedSource) | BufferedSource.asResponseBody() |

import java.util.UUID
import kotlin.test.assertFailsWith
import okhttp3.Headers.Companion.headersOf
import okhttp3.HttpUrl.Companion.toHttpUrl
import okhttp3.MediaType.Companion.toMediaType
import okhttp3.RequestBody.Companion.asRequestBody
import okhttp3.RequestBody.Companion.toRequestBody
import okhttp3.internal.EmptyTags
import okio.Buffer
import okio.ByteString.Companion.decodeHex
import okio.ByteString.Companion.encodeUtf8
import okio.GzipSource

    import this and we want to ease upgrades for their users.


## Version 4.0.0-RC2

_2019-06-21_

 *  New: Require Kotlin 1.3.40.
 *  New: Change the Kotlin API from `File.toRequestBody()` to `File.asRequestBody()` and
    `BufferedSource.toResponseBody()` to `BufferedSource.asResponseBody()`. If the returned value
    is a view of what created it, we use _as_.
 *  Fix: Permit response codes of zero for compatibility with OkHttp 3.x.

    as the API to read response trailers is unchanged. Testers may use this new stable API to
    supply trailers for a `Response`.

 *  New: `Path.asRequestBody()` is now a non-experimental API.

 *  New: `FileDescriptor.toRequestBody()` is now a non-experimental API.

 *  New: Stop using experimental coroutines APIs in our `okhttp-coroutines` artifact.