Show correct completion list when type special character in query string

## Expected Behavior
Show value completion list when user types special character in the middle of the value string

## Current Behavior
When the user types f.e. `source: example.e` instead of the value list the field name list is shown.

## Possible Solution

## Steps to Reproduce (for bugs)

1. type source in the query search

2. choose `example.org` from the list
3. delete 2 characters from the end (`example.org`)
4. the field name completion list is shown

## Context

## Your Environment

* Graylog Version:
* Java Version:
* Elasticsearch Version:
* MongoDB Version:
* Operating System:
* Browser version: