Error in Getting Extractor's Order using REST API

# Error in Getting Extractor's Order using REST API
### Reopen https://github.com/Graylog2/graylog2-server/issues/2373
I added the 2 extractors in an input. Although they need to have the order, It seems like the order doesn't exist when I get this result using Graylog REST API.

## Expected Behavior
### Result of Graylog REST API (Get Extractors)
Extractor 1
- order : 0

Extractor 2
- order : 1

## Current Behavior

### Result of Graylog REST API (Get Extractors)
Extractor 1
- order : 0

Extractor 2
- order : 0

## Steps to Reproduce (for bugs)

1. Create Input 1(Kafka Raw/Plaintext)
2. Create Extractor 1(JSON Parsing)
3. Create Extractor 2(Grok) for kafka's defult message field
4. Get Extractors of input 1 using Graylog REST API

## Context

In the process of automatically deploying an environment created through GUI through REST API, no order was specified, resulting in no desired results.

## Your Environment

* Graylog Version: 3.1.4
* Elasticsearch Version: 6.8.2
* MongoDB Version: 3
* Operating System: macOS Catalina 10.15.3
* Browser version: Google Chrome 81.0.4044.122