New Feature added and bug fix

1. add a ThreadLocal in RequestPreprocessor to expose current proxied method;
2. change HttpResponse to an interface rather than class;
3. support set a default value for url placeholder;
4. default timeout set to 5s rather that 30s;
5. upgrade fastjson version;
6. add NotResultBean to ignore ResultBeanResponseProcessor;
7. fix the ineffectiveness when change url on custom RequestPreprocessor;

Declaration of incompatibility:

• HttpResponse now is an interface, if you has directly operated this class, you should change your code.