=str #21395 fix default flushing in Deflate/Gzip compression

This was accidentally lost during the move from akka-http.

Otherwise, it isn't possible to decompress chunk-by-chunk because the
implementation will internally buffer data.

akka-stream/src/main/scala/akka/stream/impl/io/compression/CompressionUtils.scala

@@ -22,7 +22,7 @@ private[stream] object CompressionUtils {
           val compressor = newCompressor()
 
           override def onPush(): Unit = {
-            val data = compressor.compress(grab(in))
+            val data = compressor.compressAndFlush(grab(in))
             if (data.nonEmpty) push(out, data)
             else pull(in)
           }