Introduces isDuplex method to RequestBody

okhttp-testing-support/src/main/java/okhttp3/internal/duplex/AsyncRequestBody.java

@@ -26,7 +26,7 @@ import okio.BufferedSink;
 import static junit.framework.TestCase.assertTrue;
 
 /** A duplex request body that keeps the provided sinks so they can be written to later. */
-public final class AsyncRequestBody extends RequestBody implements DuplexRequestBody {
+public final class AsyncRequestBody extends RequestBody {
   private final BlockingQueue<BufferedSink> requestBodySinks = new LinkedBlockingQueue<>();
 
   @Override public @Nullable MediaType contentType() {
@@ -37,6 +37,10 @@ public final class AsyncRequestBody extends RequestBody implements DuplexRequest
     requestBodySinks.add(sink);
   }
 
+  @Override public boolean isDuplex() {
+    return true;
+  }
+
   public BufferedSink takeSink() throws InterruptedException {
     BufferedSink result = requestBodySinks.poll(5, TimeUnit.SECONDS);
     if (result == null) throw new AssertionError("no sink to take");