Fix RuntimeTypeAdapterFactory

Trying to use this class as is results in the type-property not being serialized into the JSON, thus it is not present on deserialization.
The fix from #712 (comment) works. No idea why this is not merged yet.

extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java

@@ -205,7 +205,7 @@ public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type) {
 
   @Override
   public <R> TypeAdapter<R> create(Gson gson, TypeToken<R> type) {
-    if (type.getRawType() != baseType) {
+    if (null == type || !baseType.isAssignableFrom(type.getRawType())) {
       return null;
     }