Transparent support for Brotli content-encoding.

Brotli content-encoding is supported by the major browsers, and it looks like there is pretty good support on the web server side too.

It could be supported transparently like gzip is, so that when OkHttp receives a request with the header Content-Encoding: br OkHttp uses Brotli decompression transparently.

Brotli content-encoding has reduced our payload size by about 20% for the average response.

[yschimke]

@swankjesse anything obviously wrong with this impl?

n.b. Consider this particular trivial code as public domain if you want to make a java library on github for it.

https://github.com/yschimke/oksocial/pull/295/files

package com.baulsupp.oksocial.brotli

import okhttp3.Interceptor
import okhttp3.Response
import okhttp3.ResponseBody
import okio.Okio
import org.brotli.dec.BrotliInputStream

/**
 * Transparent Brotli response support.
 *
 * Adds Accept-Encoding: br to existing encodings and checks (and strips) for Content-Encoding: br in responses
 */
object BrotliInterceptor : Interceptor {
  override fun intercept(chain: Interceptor.Chain): Response {
    val request = chain.request().newBuilder().addHeader("Accept-Encoding", "br").build()

    val response = chain.proceed(request)

    if (response.header("Content-Encoding") == "br") {
      val body = response.body()!!
      val decompressedSource = Okio.buffer(Okio.source(BrotliInputStream(body.source().inputStream())))
      return response.newBuilder()
              .removeHeader("Content-Encoding")
              .body(ResponseBody.create(body.contentType(), -1, decompressedSource))
              .build()
    }

    return response
  }
}

[swankjesse]

We shipped this in 4.1!