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

@@ -16,10 +16,6 @@
 
 package com.google.gson.typeadapters;
 
-import java.io.IOException;
-import java.util.LinkedHashMap;
-import java.util.Map;
-
 import com.google.gson.Gson;
 import com.google.gson.JsonElement;
 import com.google.gson.JsonObject;
@@ -30,6 +26,10 @@
 import com.google.gson.reflect.TypeToken;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.util.LinkedHashMap;
+import java.util.Map;
+
 
 /**
  * Adapts values whose runtime type may differ from their declaration type. This
@@ -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 (type == null || !baseType.isAssignableFrom(type.getRawType())) {
       return null;
     }