Check it out:
```
$ curl -u elastic:password -HContent-Type:application/json -XPOST localhost:9200/test/_update/foo?pretty -d'{
  "dac": {}
}'

{
  "error" : {
    "root_cause" : [
      {
        "type" : "x_content_parse_exception",
        "reason" : "[2:3] [UpdateRequest] unknown field [dac] did you mean [doc]?"
      }
    ],
    "type" : "x_content_parse_exception",
    "reason" : "[2:3] [UpdateRequest] unknown field [dac] did you mean [doc]?"
  },
  "status" : 400
}
```

The tricky thing about implementing this is that x-content doesn't
depend on Lucene. So this works by creating an extension point for the
error message using SPI. Elasticsearch's server module provides the
"spell checking" implementation.