"Export as CSV" should follow sorting of the gui

## Expected Behavior
Exported data via "Export as CSV" should look like the same as configured in the search gui.

## Current Behavior
Exported data via "Export as CSV" is not sorted by anything. I think the only sort is the physical write to the disk sequence.
So my timestamp will be exported like this:
```
2019-08-22T12:34:59.883Z
2019-08-22T12:35:00.320Z
2019-08-22T12:35:00.336Z
2019-08-22T12:35:00.717Z
2019-08-22T12:35:00.628Z
2019-08-22T12:34:17.097Z
2019-08-22T12:34:17.202Z
2019-08-22T12:34:28.285Z
2019-08-22T12:34:50.488Z
```

## Context
The logs should be sorted by timestamp per default (as it is in the GUI, this counts more for the API itself) or when the user sort for example for the source field then this should be exported as the user see the data.

## Your Environment
* Graylog Version: Graylog 3.1.0 (Oracle Corporation 1.8.0_222 on Linux 4.9.0-9-amd64)
* Elasticsearch Version: 6.8.2
* MongoDB Version: 4.0.12
* Operating System: Debian 9
* Browser version: Firefox 68

//edit: Shortened to sorting problem. The field export is not a real issue, was my fault.