@encodedName don't support "json" as mime type

[pshao25]

packages\extensions\openapi-to-typespec\test\arm-sphere is an example, it generates

@@encodedName(Catalogs.listDeviceGroups::parameters.body,
  "json",
  "listDeviceGroupsRequest"
);

which is expected to be

@@encodedName(Catalogs.listDeviceGroups::parameters.body,
  "application/json",
  "listDeviceGroupsRequest"
);