update asio2 to 2.9

[heheda123123]

之前是因为很久没发版本了才用的commit

[heheda123123]

有两类报错，一类是这个

https://github.com/zhllxt/asio2/blob/06696141283275b9d52bc7f408da53f19ef5597d/include/asio2/http/detail/flex_body.hpp#L84

另一个问题是cross里面，spdlog咋链接有问题

[Issues-translate-bot]

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There are two types of error reports, one is this

But there is no problem with the source code. Why does it become a read-only object?
https://github.com/zhllxt/asio2/blob/06696141283275b9d52bc7f408da53f19ef5597d/include/asio2/http/detail/flex_body.hpp#L84

Another problem is that there is a problem with the link in cross

[star-hengxing]

不用截图，拷贝关键报错信息过来就行

1. readonly 是因为成员函数用 const 修饰，是不能修改成员变量的
2. undefined reference to std::__atomic_futex_unsigned_base richfelker/musl-cross-make#79

[heheda123123]

不用截图，拷贝关键报错信息过来就行

1. readonly 是因为成员函数用 const 修饰，是不能修改成员变量的
2. undefined reference to std::__atomic_futex_unsigned_base richfelker/musl-cross-make#79

不过为啥有些又能编译通过呢

[heheda123123]

spdlog在cross 环境下好像炸了
其他平台测试都没问题了

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spdlog seems to explode in cross environment

[heheda123123]

这个也没问题了吧？

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Is this no problem?