According with some official text, "you dont need say what it will be encoded, because JSON is ALWAYS UTF-8"
1) try use only : "application/json" <----
2) dont mix two "charset" when using JSON, or be, the "input text (sending) should be a UTF-8", then the "output (receiving) it will be a UTF-8"!!! .... Server accept "JSON" .... Client send "JSON"
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JSON must be encoded by UTF-8, and there is no "charset" parameter.
RFC 8259 :
IANA Considerations The media type for JSON text is application/json. ... Note: No "charset" parameter is defined for this registration. Adding one really has no effect on compliant recipients.
Also,
8.1. Character Encoding
JSON text exchanged between systems that are not part of a closed ecosystem MUST be encoded using UTF-8 [RFC3629].
Previous specifications of JSON have not required the use of UTF-8 when transmitting JSON text. However, the vast majority of JSON-based software implementations have chosen to use the UTF-8 encoding, to the extent that it is the only encoding that achieves interoperability.
Implementations MUST NOT add a byte order mark (U+FEFF) to the beginning of a networked-transmitted JSON text. In the interests of interoperability, implementations that parse JSON texts MAY ignore the presence of a byte order mark rather than treating it as an error.
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The higher the degree, the greater the respect given to the humblest!RAD 11.3