fix charset

[synodriver]

Current implementation ignores the charset field in the response class, which may lead to some problem, while starlette handles that properly

So we can reuse starlette's code to set proper content-type and charset, like changing this line.

[synodriver]

With this patch, the proper charset is set like the base class in starlette

[sysid]

Thanks @synodriver, will merge.