Fixes for #46

Author: Caitlin Bales (MSFT)

graphsdk/src/main/java/com/microsoft/graph/serializer/EnumSetSerializer.java

@@ -59,7 +59,7 @@ public static EnumSet deserialize(Type type, String jsonStrToDeserialize) {
      * @param src The source EnumSet
      * @return A comma-delimited string of enum values
      */
-    public static JsonPrimitive serialize(EnumSet src) {
+    public static JsonPrimitive serialize(EnumSet<?> src) {
         String serializedString = "";
 
         Iterator i = src.iterator();

graphsdk/src/main/java/com/microsoft/graph/serializer/GsonFactory.java

@@ -162,7 +162,7 @@ public DateOnly deserialize(final JsonElement json,
             public JsonElement serialize(final EnumSet src,
                                          final Type typeOfSrc,
                                          final JsonSerializationContext context) {
-                if (src == null) {
+                if (src == null || src.size() == 0) {
                     return null;
                 }