fixed serialization problems for mime messages

Author: Markus Mertens

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);
             }