Use existing character set in POST body when possible

ReactAndroid/src/main/java/com/facebook/react/modules/network/NetworkingModule.java

@@ -372,7 +372,13 @@ public void onProgress(long bytesWritten, long contentLength, boolean done) {
           return;
         }
       } else {
-        requestBody = RequestBody.create(contentMediaType, body.getBytes(StandardCharsets.UTF_8));
+        // Use getBytes() to convert the body into a byte[], preventing okhttp from
+        // appending the character set to the Content-Type header when otherwise unspecified
+        // https://github.com/facebook/react-native/issues/8237
+        Charset charset = contentMediaType == null
+          ? StandardCharsets.UTF_8
+          : contentMediaType.charset(StandardCharsets.UTF_8);
+        requestBody = RequestBody.create(contentMediaType, body.getBytes(charset));
       }
     } else if (data.hasKey(REQUEST_BODY_KEY_BASE64)) {
       if (contentType == null) {

ReactAndroid/src/test/java/com/facebook/react/modules/network/NetworkingModuleTest.java

@@ -18,6 +18,7 @@
 import com.facebook.react.bridge.ReactContext;
 import com.facebook.react.bridge.WritableArray;
 import com.facebook.react.bridge.WritableMap;
+import com.facebook.react.common.StandardCharsets;
 import com.facebook.react.common.network.OkHttpCallUtil;
 import com.facebook.react.modules.core.DeviceEventManagerModule.RCTDeviceEventEmitter;
 
@@ -346,6 +347,94 @@ public Object answer(InvocationOnMock invocation) throws Throwable {
     assertThat(argumentCaptor.getValue().body().contentType().toString()).isEqualTo("application/json");
   }
 
+  @Test
+  public void testRespectsExistingCharacterSet() throws Exception {
+    RCTDeviceEventEmitter emitter = mock(RCTDeviceEventEmitter.class);
+    ReactApplicationContext context = mock(ReactApplicationContext.class);
+    when(context.getJSModule(any(Class.class))).thenReturn(emitter);
+
+    OkHttpClient httpClient = mock(OkHttpClient.class);
+    when(httpClient.newCall(any(Request.class))).thenAnswer(new Answer<Object>() {
+      @Override
+      public Object answer(InvocationOnMock invocation) throws Throwable {
+        Call callMock = mock(Call.class);
+        return callMock;
+      }
+    });
+    OkHttpClient.Builder clientBuilder = mock(OkHttpClient.Builder.class);
+    when(clientBuilder.build()).thenReturn(httpClient);
+    when(httpClient.newBuilder()).thenReturn(clientBuilder);
+    NetworkingModule networkingModule = new NetworkingModule(context, "", httpClient);
+
+    JavaOnlyMap body = new JavaOnlyMap();
+    body.putString("string", "Friðjónsson");
+
+    mockEvents();
+
+    networkingModule.sendRequest(
+      "POST",
+      "http://somedomain/bar",
+      0,
+      JavaOnlyArray.of(JavaOnlyArray.of("Content-Type", "text/plain; charset=utf-16")),
+      body,
+      /* responseType */ "text",
+      /* useIncrementalUpdates*/ true,
+      /* timeout */ 0,
+      /* withCredentials */ false);
+
+    ArgumentCaptor<Request> argumentCaptor = ArgumentCaptor.forClass(Request.class);
+    verify(httpClient).newCall(argumentCaptor.capture());
+
+    Buffer contentBuffer = new Buffer();
+    argumentCaptor.getValue().body().writeTo(contentBuffer);
+    assertThat(contentBuffer.readString(StandardCharsets.UTF_16)).isEqualTo("Friðjónsson");
+  }
+
+  @Test
+  public void testGracefullyRecoversFromInvalidContentType() throws Exception {
+    RCTDeviceEventEmitter emitter = mock(RCTDeviceEventEmitter.class);
+    ReactApplicationContext context = mock(ReactApplicationContext.class);
+    when(context.getJSModule(any(Class.class))).thenReturn(emitter);
+
+    OkHttpClient httpClient = mock(OkHttpClient.class);
+    when(httpClient.newCall(any(Request.class))).thenAnswer(new Answer<Object>() {
+      @Override
+      public Object answer(InvocationOnMock invocation) throws Throwable {
+        Call callMock = mock(Call.class);
+        return callMock;
+      }
+    });
+    OkHttpClient.Builder clientBuilder = mock(OkHttpClient.Builder.class);
+    when(clientBuilder.build()).thenReturn(httpClient);
+    when(httpClient.newBuilder()).thenReturn(clientBuilder);
+    NetworkingModule networkingModule = new NetworkingModule(context, "", httpClient);
+
+    JavaOnlyMap body = new JavaOnlyMap();
+    body.putString("string", "test");
+
+    mockEvents();
+
+    networkingModule.sendRequest(
+      "POST",
+      "http://somedomain/bar",
+      0,
+      JavaOnlyArray.of(JavaOnlyArray.of("Content-Type", "invalid")),
+      body,
+      /* responseType */ "text",
+      /* useIncrementalUpdates*/ true,
+      /* timeout */ 0,
+      /* withCredentials */ false);
+
+    ArgumentCaptor<Request> argumentCaptor = ArgumentCaptor.forClass(Request.class);
+    verify(httpClient).newCall(argumentCaptor.capture());
+
+    Buffer contentBuffer = new Buffer();
+    argumentCaptor.getValue().body().writeTo(contentBuffer);
+
+    assertThat(contentBuffer.readString(StandardCharsets.UTF_8)).isEqualTo("test");
+    assertThat(argumentCaptor.getValue().header("Content-Type")).isEqualTo("invalid");
+  }
+
   @Test
   public void testMultipartPostRequestSimple() throws Exception {
     PowerMockito.mockStatic(RequestBodyUtil.class);