Only allow a single content-type header to be applied while using googlehttpclient (#1737)

* Only allow a single content-type header to be applied while using google client

* Run code formatter

* Run formatter again via cmd line

Co-authored-by: Marvin Froeder <velo@users.noreply.github.com>

googlehttpclient/src/main/java/feign/googlehttpclient/GoogleHttpClient.java

@@ -87,7 +87,12 @@ public class GoogleHttpClient implements Client {
     // Setup headers
     final HttpHeaders headers = new HttpHeaders();
     for (final Map.Entry<String, Collection<String>> header : inputRequest.headers().entrySet()) {
-      headers.set(header.getKey(), header.getValue());
+      // We already set the Content-Type header via ByteArrayContent
+      // Content-Type is defined as a singleton field
+      // https://www.rfc-editor.org/rfc/rfc9110.html#section-8.3-7
+      if (!header.getKey().equals("Content-Type")) {
+        headers.set(header.getKey(), header.getValue());
+      }
     }
     // Some servers don't do well with no Accept header
     if (inputRequest.headers().get("Accept") == null) {

googlehttpclient/src/test/java/feign/googlehttpclient/GoogleHttpClientTest.java

@@ -15,7 +15,16 @@ package feign.googlehttpclient;
 
 import feign.Feign;
 import feign.Feign.Builder;
+import feign.Response;
+import feign.Util;
+import feign.assertj.MockWebServerAssertions;
 import feign.client.AbstractClientTest;
+import okhttp3.mockwebserver.MockResponse;
+import org.junit.Test;
+import java.util.Collections;
+import static feign.Util.UTF_8;
+import static org.assertj.core.api.Assertions.entry;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assume.assumeFalse;
 
 public class GoogleHttpClientTest extends AbstractClientTest {
@@ -55,4 +64,20 @@ public class GoogleHttpClientTest extends AbstractClientTest {
     assumeFalse("Google HTTP client client do not support gzip compression", false);
   }
 
+  @Test
+  public void testContentTypeHeaderGetsAddedOnce() throws Exception {
+    server.enqueue(new MockResponse()
+        .setBody("AAAAAAAA"));
+    TestInterface api = newBuilder()
+        .target(TestInterface.class, "http://localhost:" + server.getPort());
+
+    Response response = api.postWithContentType("foo", "text/plain");
+    // Response length should not be null
+    assertEquals("AAAAAAAA", Util.toString(response.body().asReader(UTF_8)));
+
+    MockWebServerAssertions.assertThat(server.takeRequest())
+        .hasHeaders(entry("Content-Type", Collections.singletonList("text/plain")),
+            entry("Content-Length", Collections.singletonList("3")))
+        .hasMethod("POST");
+  }
 }