Leaving colon (:) in a stream rule passes test against stream but doesnt route message

## Expected Behavior

Having a colon in a stream rule should not pass the "test against stream" for a message as this isn't valid syntax

## Current Behavior

If a stream rule field name ends in a colon (:) messages that match the intended rule are not routed to the stream, However if you run "test against stream" for a message that would match the intended rule it shows as passing the rule.

## Possible Solution

Check for / don't allow a colon in the field section of a rule.

## Steps to Reproduce (for bugs)

1. Create a new stream
2. Create a rule on that stream ending the Field name with a colon
3. Receive messages matching the intended rule, they will not route to the stream
4. Located a message in "all messages" and "test against stream" will return as true

## Context

Confuses and slowed troubleshooting a new steam rule (user error)

## Your Environment

* Graylog Version: 2.4.4
* Elasticsearch Version: 5.6.3
* Operating System: Ubuntu OVA virtual appliance