fix deserialize

src/main/java/cn/montaro/aria2/client/http/Aria2HttpProxy.java

@@ -79,11 +79,20 @@ public class Aria2HttpProxy implements InvocationHandler {
     }
 
     private Object deserialize(String json, Type resultType) {
-        if (resultType.equals(String.class)) {
-            return json;
-        }
         JsonObject jsonObject = JsonParser.parseString(json).getAsJsonObject();
         JsonElement result = jsonObject.get("result");
+
+        if (Objects.isNull(result)) {
+            return null;
+        }
+
+        if (resultType.equals(String.class)) {
+            if (result.isJsonPrimitive()) {
+                return result.getAsJsonPrimitive().getAsString();
+            }
+            return json;
+        }
+
         return gson.fromJson(result, resultType);
     }