When trying to convert hash to string I get failed action with response of 400, dropping action:

**logstash version = 1.5.4**
**ES_version = 1.7.2**
**java_version = 1.8**
Hi there ,
I am using the output plugin for elasticsearch for a json log. I successfully parse the json part of the log like this

```
json {
source => "json"
}
```

Since the json has a nested json within a field name **params** I get something like this in kibana:
**example json message**

``` json
{"method”:”data”,”path":"data","format":"json","controller":"data","action":"data","status": data,"duration": data,"view": data,"db": data,"params":{"shortname":"data","format":"json"},"@timestamp":"2015-10-14T15:38:47.301Z","@version":"1","message":"[200] GET data (data#data)"}
```

**example field in kibana**

```
params.value.value.value.value = value
```

Since I don't need the param field to parsed as json I want to convert it to string and because that's not possible with a mutate convert command so I tried the following and with all combinations I get the above mentioned error in logstash log file and the log is dropped.

```
mutate {
update => { "params" => "%{params}" }
}
```

```
ruby {
code => "event['params'] = event['params'].to_s"
}
```

There's something in the log that ES output plugin or ES itself doesn't like after I try to convert the params json to string which drops the whole batch

Thanks!