fixed serialization problems for mime messages

src/main/java/com/microsoft/graph/http/CoreHttpProvider.java

@@ -296,7 +296,15 @@ public <Result, Body> Request getHttpRequest(@Nonnull final IHttpRequest request
 			}
 		} else {
 			logger.logDebug("Sending " + serializable.getClass().getName() + " as request body");
-			final String serializeObject = serializer.serializeObject(serializable);
+
+			String serializeObject = null;
+
+			if ("text/plain".equals(contenttype) && serializable instanceof String) {
+				serializeObject = (String)serializable;
+			} else {
+				serializeObject = serializer.serializeObject(serializable);
+			}
+
             if(serializeObject == null) {
                 throw new ClientException("Error during serialization of request body, the result was null", null);
             }

src/main/java/com/microsoft/graph/serializer/DerivedClassIdentifier.java

@@ -8,12 +8,16 @@
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
 
+/** This class provides methods to get the derived class corresponding to the OData type when deserializing payloads. */
 public class DerivedClassIdentifier {
 
     private final static String ODATA_TYPE_KEY = "@odata.type";
 
     private final ILogger logger;
-
+    /**
+     * Creates a new instance of the dereived class identifier.
+     * @param logger The logger to use.
+     */
     public DerivedClassIdentifier(@Nonnull ILogger logger) {
         this.logger = Objects.requireNonNull(logger, "logger parameter cannot be null");;
     }

src/test/java/com/microsoft/graph/http/CoreHttpProviderTests.java

@@ -20,6 +20,7 @@
 import com.microsoft.graph.serializer.DefaultSerializer;
 import com.microsoft.graph.serializer.ISerializer;
 
+import okio.Buffer;
 import org.junit.jupiter.api.Test;
 
 import java.io.ByteArrayInputStream;
@@ -39,6 +40,8 @@
 import okhttp3.Request;
 import okhttp3.Response;
 import okhttp3.ResponseBody;
+import okhttp3.RequestBody;
+import org.mockito.Mockito;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
@@ -49,6 +52,8 @@
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 
 public class CoreHttpProviderTests {
@@ -288,4 +293,36 @@ public void getHttpRequestSetsRetryOrRedirectOptionsOnNonDefaultValues() throws
 
         assertNotNull(retryOptions);
     }
+
+    @Test
+    public void getHttpRequestWithTextPlainBodyDoesNotSerializeAsJson() throws IOException {
+        final IHttpRequest absRequest = mock(IHttpRequest.class);
+        when(absRequest.getRequestUrl()).thenReturn(new URL("https://graph.microsoft.com/v1.0/me"));
+        when(absRequest.getHttpMethod()).thenReturn(HttpMethod.POST);
+        final ISerializer serializer = mock(ISerializer.class);
+        final ILogger logger = mock(ILogger.class);
+
+        mProvider = new CoreHttpProvider(serializer,
+            logger,
+            new OkHttpClient.Builder().build());
+
+        // GIVEN: A "text/plain" request body
+        final HeaderOption option = new HeaderOption("Content-Type", "text/plain");
+        when(absRequest.getHeaders()).thenReturn(Arrays.asList(option));
+        final String expectedBody = "Plain String Body";
+
+        //WHEN: getHttpRequest is called
+        final Request request = mProvider.getHttpRequest(absRequest, String.class, expectedBody);
+
+        // THEN: The serializer must not be called
+        verify(serializer, never()).serializeObject(Mockito.any());
+
+        // AND: We expect the request body to contain the plain String, not serialized as Json
+        final Buffer buffer = new Buffer();
+        final RequestBody requestBody = request.body();
+        assertNotNull(requestBody);
+        requestBody.writeTo(buffer);
+        final String actualRequestBody = buffer.readUtf8();
+        assertEquals(expectedBody, actualRequestBody);
+    }
 }