[Logging] Debug output not helpful if message field is removed

While debugging something, I ran across this logging output:

```
19:19:13.544 [[main]>worker3] DEBUG logstash.filters.json - Event after json filter {:event=>2017-04-23T23:19:12.831Z 10.231.2.82 %{message}}
19:19:13.544 [[main]>worker3] DEBUG logstash.filters.grok - Running grok filter {:event=>2017-04-23T23:19:12.831Z 10.231.2.82 %{message}}
19:19:13.545 [[main]>worker3] DEBUG logstash.filters.grok - Event now: {:event=>2017-04-23T23:19:12.831Z 10.231.2.82 %{message}}
```

Fortunately this was unrelated to my issue, but it certainly would have been frustrating if I were actually looking at what Grok was up too. For reference, this is my Grok filter:

```ruby
grok {
match => [ "request", "%{WORD:http_method} %{URIPATH:uri} HTTP/1.1" ]
}
```