Events' metadata hash is overwritten by headers and properties

[magnusbaeck]

If add_field is used to add metadata fields, e.g. like

input {
  rabbitmq {
    ...
    add_field => {
      "[@metadata][indexType]" => "myindex"
    }
  }
}

those fields won't end up in the events produced by the input. This is a regression introduced in #83 when

event["@metadata"]["rabbitmq_headers"] = get_headers(metadata)
event["@metadata"]["rabbitmq_properties"] = get_properties(metadata)

was replaced by this:

meta = {
  "rabbitmq_headers" => get_headers(metadata),
   "rabbitmq_properties" => get_properties(metadata)
}
event.set("@metadata", meta)

This bug was originally reported here: https://discuss.elastic.co/t/rabbitmq-input-plugin-add-field-doesnt-write-metadata/82152