different output from io.FlowReader and io.FlowWrite

[rgre26]

Goal:
Reduce size of mitmdump file, by removing large response entries.

Code:

from mitmproxy import io

with open(file,'rb') as file_in:
    fr = io.FlowReader(file_in)
    with open(f'{file}.out', 'ab') as file_out:
        fw = io.FlowWriter(file_out)
            for n,f in enumerate(fr.stream()):
                fw.add(f)

Got a new file, but with different sha256, so i used diff:

1,4c1
< 9112:4:type;
< 4:http;
< 7:version;
< 2:18#9:websocket;
---
> 9052:9:websocket;
35c32,33
< 8:HTTP/2.0,}17:timestamp_created;
---
> 8:HTTP/2.0,}6:backup;
> 0:~17:timestamp_created;
44,46c42,54
< 6313:4:via2;
< 0:~11:cipher_list;
< 0:]11:cipher_name;
---
> 6277:3:via;
> 0:~19:timestamp_tcp_setup;
> 17:1692359990.453262^7:address;
> 47:37:firefox.settings.services.mozilla.com;
> 3:443#]19:timestamp_tls_setup;
> 18:1692359990.4875603^13:timestamp_end;
> 0:~15:timestamp_start;
> 18:1692359990.4108424^3:sni;
> 37:firefox.settings.services.mozilla.com;
> 11:tls_version;
> 7:TLSv1.3;
> 11:cipher_list;
> 0:]6:cipher;
49c57,58
< 16:2:h2,8:http/1.1,]16:certificate_list;
---
> 16:2:h2,8:http/1.1,]4:alpn;
> 2:h2,16:certificate_list;

1. Is it normal, that generated output diffs original?
2. If yes, is it possible to keep old file format when rewriting file?

[mhils]

I can't really tell from the diff: is the flow the same with just reordered keys, or are we loosing information? The latter would be a bug we should fix. The former is less urgent, but something I'd be happy to review a PR for nonetheless, at least if it is possible to fix without adding complexity.