Test that duplex works fine with interceptors

okhttp-testing-support/src/main/java/okhttp3/UppercaseRequestInterceptor.java

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+/*
+ * Copyright (C) 2019 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package okhttp3;
+
+import java.io.IOException;
+import okio.Buffer;
+import okio.BufferedSink;
+import okio.ByteString;
+import okio.ForwardingSink;
+import okio.Okio;
+import okio.Sink;
+
+/** Rewrites the request body sent to the server to be all uppercase. */
+public final class UppercaseRequestInterceptor implements Interceptor {
+  @Override public Response intercept(Chain chain) throws IOException {
+    return chain.proceed(uppercaseRequest(chain.request()));
+  }
+
+  /** Returns a request that transforms {@code request} to be all uppercase. */
+  private Request uppercaseRequest(Request request) {
+    RequestBody uppercaseBody = new ForwardingRequestBody(request.body()) {
+      @Override public void writeTo(BufferedSink sink) throws IOException {
+        delegate().writeTo(Okio.buffer(uppercaseSink(sink)));
+      }
+    };
+    return request.newBuilder()
+        .method(request.method(), uppercaseBody)
+        .build();
+  }
+
+  private Sink uppercaseSink(Sink sink) {
+    return new ForwardingSink(sink) {
+      @Override public void write(Buffer source, long byteCount) throws IOException {
+        ByteString bytes = source.readByteString(byteCount);
+        delegate().write(new Buffer().write(bytes.toAsciiUppercase()), byteCount);
+      }
+    };
+  }
+}