Conditionals: no way to check if boolean field exists

In the Logstash config language, I can't determine any way to check if a boolean field exists.

e.g. Using this conditional
`if[myfield] { ...exists... } else { ...doesn't exist... }`
The results from testing this conditional statement are:

```
[myfield] does not exist --> false
[myfield] exists, is true --> true
[myfield] exists, is false --> false //expected true because the field exists
```

I've asked the question here as well to reach out and see if someone else knows a way to check for boolean existence.
http://stackoverflow.com/questions/26287082/logstash-config-check-if-boolean-field-exists