Avoid AttributeError when key or value are already decoded

[sarah-github]

I got the error:

File "/usr/bin/mitmproxy2swagger", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/usr/lib/python3.12/site-packages/mitmproxy2swagger/mitmproxy2swagger.py", line 265, in main
    body_val[key.decode("utf-8")] = value.decode("utf-8")
                                    ^^^^^^^^^^^^
AttributeError: 'str' object has no attribute 'decode'. Did you mean: 'encode'?

and fixed it by the current change.

[atlekbai]

Encountered the same error, I can assure that this fixes the error 💯

Thanks, @sarah-github