[OLINGO-1114] Test + fix for NULL value type

lib/client-core/src/main/java/org/apache/olingo/client/core/serialization/JsonSerializer.java

@@ -357,7 +357,7 @@ public class JsonSerializer implements ODataSerializer {
       String type = valuable.getType();
       if ((!valuable.isCollection() &&
           (type == null || type.isEmpty()) &&
-          valuable.isPrimitive()) || valuable.isNull()) {
+          valuable.isPrimitive())) {
         type = EdmPrimitiveTypeKind.String.getFullQualifiedName().toString();
       }
       if (type != null && !type.isEmpty() && !isODataMetadataNone) {

lib/client-core/src/test/java/org/apache/olingo/client/core/serialization/JsonSerializerTest.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2018 The Apache Software Foundation.
+ *
+ * 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 org.apache.olingo.client.core.serialization;
+
+import java.io.StringWriter;
+import org.apache.olingo.client.api.ODataClient;
+import org.apache.olingo.client.api.domain.ClientEntity;
+import org.apache.olingo.client.api.domain.ClientObjectFactory;
+import org.apache.olingo.client.api.serialization.ODataSerializerException;
+import org.apache.olingo.client.core.ODataClientFactory;
+import org.apache.olingo.commons.api.edm.FullQualifiedName;
+import org.apache.olingo.commons.api.format.ContentType;
+import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertThat;
+import org.junit.Test;
+
+public class JsonSerializerTest {
+        
+    @Test
+    public void testClientEntityJSONWithNull() throws ODataSerializerException {
+        String expectedJson = "{\"@odata.type\":\"#test.testClientEntity\","
+                + "\"testInt32@odata.type\":\"Int32\","
+                + "\"testInt32\":12,"
+                + "\"testInt32Null@odata.type\":\"Int32\""
+                + ",\"testInt32Null\":null,"
+                + "\"testString@odata.type\":\"String\","
+                + "\"testString\":\"testString\","
+                + "\"testStringNull@odata.type\":\"String\","
+                + "\"testStringNull\":null}";
+        
+        ODataClient odataClient = ODataClientFactory.getClient();
+        ClientObjectFactory objFactory = odataClient.getObjectFactory();
+        ClientEntity clientEntity = objFactory.newEntity(new FullQualifiedName("test", "testClientEntity"));
+        
+        clientEntity.getProperties().add(
+                objFactory.newPrimitiveProperty(
+                        "testInt32",
+                        objFactory.newPrimitiveValueBuilder().buildInt32(12)
+                )
+        );
+        clientEntity.getProperties().add(
+                objFactory.newPrimitiveProperty(
+                        "testInt32Null",
+                        objFactory.newPrimitiveValueBuilder().buildInt32(null)
+                )
+        );
+        clientEntity.getProperties().add(
+                objFactory.newPrimitiveProperty(
+                        "testString",
+                        objFactory.newPrimitiveValueBuilder().buildString("testString")
+                )
+        );
+        clientEntity.getProperties().add(
+                objFactory.newPrimitiveProperty(
+                        "testStringNull",
+                        objFactory.newPrimitiveValueBuilder().buildString(null)
+                )
+        );
+        
+        JsonSerializer jsonSerializer = new JsonSerializer(false, ContentType.JSON);
+        
+        StringWriter writer = new StringWriter();
+        jsonSerializer.write(writer, odataClient.getBinder().getEntity(clientEntity));
+        assertThat(writer.toString(), is(expectedJson));
+    }
+}