Fields show up as nested JSON in Kibana

When Packetbeat exports a field of type dictionary, and the keys are not known in advance, the Discovery page in Kibana will display the field as a nested JSON object:

http.response.headers = {
        "content-length": 12,
        "content-type": "application/json"
}

To fix this you need to reload the index pattern in Kibana under the Management→Index Patterns, and the index pattern will be updated with a field for each key available in the dictionary:

http.response.headers.content-length = 12
http.response.headers.content-type = "application/json"