Cannot send boolean fields via Gelf

## Expected Behavior
When sending a GELF message with a boolean field I expect the boolean field to show up
in my message.

## Current Behavior
The boolean field will be dropped! The rest of the gelf message gets processed.

## Possible Solution
Add support like in #3974. There was discussed that then message brought in with boolean
support would lead to errors since the boolean values would have been treated as strings
prior. That is not the case, since boolean fields get dropped.

## Steps to Reproduce (for bugs)
1. Setup a GELF/HTTP input
2. `curl -X POST -H 'Content-Type: application/json' -d '{ "version": "1.1", "host": "example.org", "short_message": "A short message", "bug": true, "level": 5, "_some_info": "foo" }' 'http://localhost:12201/gelf'`
3. The `bug` field does not make it into the processed message.

Refs: #1632
Refs: #3974

## Your Environment
* Graylog Version: 3.0.0
* Elasticsearch Version: 5.6.5