Invalid default encoding without charset for json

[0xpr03]

Hello,

the current assumption is that if no charset is defined, a fallback to latin1 should be done. Problem is that for application/json no charset means utf8: https://datatracker.ietf.org/doc/html/rfc8259#section-8.1 when using something like json.decode(result.body). So right now one needs to work around flutter behaving differently for JSON payloads than any other system.

[Kaps61929]

please assign this issue to me.