compatible problem with FutureAdapter

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Environment

• Dubbo version: 2.7.6
• Operating System version: xxx
• Java version: xxx

Steps to reproduce this issue

in dubbo, we can write async code like below:

import com.alibaba.dubbo.remoting.exchange.ResponseCallback;
import com.alibaba.dubbo.remoting.exchange.ResponseFuture;
import com.alibaba.dubbo.rpc.RpcContext;
import com.alibaba.dubbo.rpc.protocol.dubbo.FutureAdapter;
import com.alibaba.dubbo.rpc.Result;

interface TestService {
    String test();
}

testService.test();
ResponseFuture future = ((FutureAdapter) RpcContext.getContext().getFuture()).getFuture();
future.setCallback(new ResponseCallback(){
    public void done(Object response){
        assert response instanceof Result;// in 2.6.x
        assert response instanceof String;// in 2.7.x
    }

    public void caught(Throwable exception){
    }
});

however, response is an object of Result in 2.6.x but String in 2.7.x.
when project update to 2.7.x, we may find runtime error.

如果我们用上面的代码段做异步调用，在2.7.x里面，response会是一个String，但是在2.6.x，response会是一个com.alibaba.dubbo.rpc.Result。
如果从2.6升级到2.7，必须要查找所有进行调用的地方并修改代码，这不太合理。