## 优化缓存括展，支持缓存null值
当前CacheFilter存在无法缓存null值问题
但是某些rpc接口被设计成可能返回空值（比如跟据用户名查询用户信息接口，如果查询不到，返回null）
## 增加一个内部类包装待缓存的数据
static class ValueWrapper {
private final Object value;
public ValueWrapper(Object value){
this.value = value;
}
public Object get() {
return this.value;
}
}
## Verifying this change
XXXXX
Follow this checklist to help us incorporate your contribution quickly and
easily:
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little better when cross module dependency exist. If the new feature or
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[ Full content available at:
https://github.com/apache/incubator-dubbo/pull/2480 ]
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