Response cache returns json object instead of my html result

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If the response is cached the cache returns an object with content-type: application/json instead of content-type: text/html;

The cached returned object looks like this:

{
  "body": "<the rendered html string>",
  "status": 200,
  "headers": {}
}

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Looks like the problem could be fixed simply by setting the property __isActionResult in the cached object of type ActionResult to true.