Stop appending the compression buffer's unused capacity to the request body

client/client-http/src/main/java/software/amazon/smithy/java/client/http/compression/Gzip.java

@@ -32,7 +32,7 @@ public DataStream compress(DataStream data) {
             var gzip = new GZIPOutputStream(bos);
             in.transferTo(gzip);
             gzip.close();
-            return DataStream.ofBytes(bos.toByteBuffer().array());
+            return DataStream.ofByteBuffer(bos.toByteBuffer(), data.contentType());
         } catch (IOException e) {
             throw new UncheckedIOException(e);
         }

client/client-http/src/test/java/software/amazon/smithy/java/client/http/compression/GzipTest.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.java.client.http.compression;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.equalTo;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.nio.charset.StandardCharsets;
+import java.util.zip.GZIPInputStream;
+import java.util.zip.GZIPOutputStream;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+import software.amazon.smithy.java.io.ByteBufferUtils;
+import software.amazon.smithy.java.io.datastream.DataStream;
+
+public class GzipTest {
+
+    private static final Gzip GZIP = new Gzip();
+
+    @ParameterizedTest
+    @ValueSource(ints = {0, 1, 3, 150, 3000, 15000})
+    public void compressedBodyIsExactlyTheGzipStream(int payloadLength) throws Exception {
+        var payload = payload(payloadLength);
+
+        var compressed = GZIP.compress(DataStream.ofBytes(payload));
+        var body = ByteBufferUtils.getBytes(compressed.asByteBuffer());
+
+        assertThat(body, equalTo(referenceGzip(payload)));
+        assertThat(compressed.contentLength(), equalTo((long) body.length));
+        assertThat(decompress(body), equalTo(payload));
+    }
+
+    @Test
+    public void preservesContentType() {
+        var compressed = GZIP.compress(DataStream.ofString("{\"hello\":\"world\"}", "application/json"));
+
+        assertThat(compressed.contentType(), equalTo("application/json"));
+    }
+
+    private static byte[] payload(int length) {
+        var pattern = "Hello World! ";
+        var sb = new StringBuilder(length);
+        while (sb.length() < length) {
+            sb.append(pattern, 0, Math.min(pattern.length(), length - sb.length()));
+        }
+        return sb.toString().getBytes(StandardCharsets.UTF_8);
+    }
+
+    private static byte[] referenceGzip(byte[] payload) throws Exception {
+        var out = new ByteArrayOutputStream();
+        try (var gzip = new GZIPOutputStream(out)) {
+            gzip.write(payload);
+        }
+        return out.toByteArray();
+    }
+
+    private static byte[] decompress(byte[] compressed) throws Exception {
+        try (var gzipIn = new GZIPInputStream(new ByteArrayInputStream(compressed))) {
+            return gzipIn.readAllBytes();
+        }
+    }
+}