OpenAPI: added charset=utf-8 to application/json content type (#1108)

* Added charset=utf-8 to application/json content type

This change is linked to https://github.com/drakkan/sftpgo/issues/1101 and should partially alleviate the need to change the content type in the files generated by openapi-generator-cli

Signed-off-by: Jon Bendtsen <github@jonb.dk>

* extra newline

Signed-off-by: Jon Bendtsen <github@jonb.dk>

* Signed-off-by: Jon Bendtsen github@jonb.dk

Signed-off-by: Jon Bendtsen github@jonb.dk
Signed-off-by: Jon Bendtsen <github@jonb.dk>

* This change is linked to #1101 and should partially alleviate the need to change the content type in the files generated by openapi-generator-cli.

Signed-off-by: Jon Bendtsen <github@jonb.dk>

Signed-off-by: Jon Bendtsen <github@jonb.dk>
Signed-off-by: Jon Bendtsen github@jonb.dk