unfiltered.response.JsonContent allows charset param which is wrong

[hamnis]

From RFC-7159:

https://tools.ietf.org/html/rfc7159#section-8

JSON text SHALL be encoded in UTF-8, UTF-16, or UTF-32. The default
encoding is UTF-8, and JSON texts that are encoded in UTF-8 are
interoperable in the sense that they will be read successfully by the
maximum number of implementations; there are many implementations
that cannot successfully read texts in other encodings (such as
UTF-16 and UTF-32).

https://tools.ietf.org/html/rfc7159#section-11

The MIME media type for JSON text is application/json.

Type name: application

Subtype name: json

Required parameters: n/a

Optional parameters: n/a

No parameters are allowed, meaning that specifying charset should not be possible.