Regex matching not work in when clause without "== true" at the end

[hc4]

Problem description

This condition doesn't match without "== true" at the end.

when
  regex(".*", to_string($message.message)).matches == true

Environment

• Graylog Version: 2.1.0-beta3
• Pipeline Processor plugin version: 1.1.0-beta3

[kroepke]

I guess this would require more aggressive resolution of return types.

As that can get arbitrarily complex, is limited to only a subset of built-in functions and has a reasonable workaround, I'd push this back.

However, since it is a useful feature I'll keep it open for now.

[kroepke]

At the very least we should return a parser error and not silently substitute false as the value here.