Unify Content-Type for JSON responses

[banool]

Hey, I see in some places in the code base, we use application/json as the Content-Type:

• poem-openapi/src/payload/json.rs

Whereas in others, we use application/json; charset=utf-8:

• poem/src/web/json.rs

I wonder is there a reason for this? If not, it'd be greater if we could return either one or the other in all cases.

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