这种方式缺点是方法名不能一样

如果A协议和B协议的一个方法名相同，
    struct objc_method_description *methods = protocol_copyMethodDescriptionList(
            httpProtocol, YES, YES, &numberOfMethods);
    for (unsigned int i = 0; i < numberOfMethods; i++) {
        struct objc_method_description method = methods[i];
        [_selToHandlerMap setValue:handler forKey:NSStringFromSelector(method.name)];
    }

这个就有问题了
