Full HTTP/2 support

### Actual behaviour
- After enabling http2 on my server (changed to php7.2-fpm) i noticed that only the android client is not yet able to speak http2

### Expected behaviour
- Like every actual browser and the Nextcloud-iOS/2.20.8 client. They work fine.

### Environment data
Android version: 6.0.1

Device model: SM-G900F (Galaxy S5)

Stock or customized system: stock

Nextcloud app version: Nextcloud-dev Ver. 20180526

Nextcloud server version: 13.0.2.1

### Logs
#### Web server log
```
192.168.188.42 - - [04/Jun/2018:10:23:13 +0200] "GET /nc/ocs/v1.php/cloud/capabilities?format=json HTTP/1.1" 200 2777 "-" "Mozilla/5.0 (Android) ownCloud-android/20180526"
192.168.188.42 - - [04/Jun/2018:10:23:13 +0200] "GET /nc/ocs/v1.php/cloud/user?format=json HTTP/1.1" 200 1198 "-" "Mozilla/5.0 (Android) ownCloud-android/20180526"
192.168.188.42 - - [04/Jun/2018:10:23:13 +0200] "PROPFIND /nc/remote.php/webdav/ HTTP/1.1" 207 1450 "-" "Mozilla/5.0 (Android) Nextcloud-android/20180526"
192.168.188.42 - - [04/Jun/2018:10:23:13 +0200] "PROPFIND /nc/remote.php/webdav/ HTTP/1.1" 207 11866 "-" "Mozilla/5.0 (Android) Nextcloud-android/20180526"
192.168.188.42 - - [04/Jun/2018:10:23:13 +0200] "GET /nc/ocs/v2.php/apps/files_sharing/api/v1/shares?path=%2F&reshares=true&subfiles=true HTTP/1.1" 200 1632 "-" "Mozilla/5.0 (Android) ownCloud-android/20180526"
```