appstream-util: <screenshot> url not found

appstream-util throws the error: ` url not found [https://doc.schlomp.space/whyblocked/whyblocked_screenshot.png]`.

The image is reachable:
```
% curl -I https://doc.schlomp.space/whyblocked/whyblocked_screenshot.png
HTTP/1.1 200 OK
Server: nginx/1.14.0
Date: Mon, 22 Oct 2018 06:38:08 GMT
Content-Type: image/png
Content-Length: 34834
Last-Modified: Mon, 22 Oct 2018 05:31:25 GMT
Connection: keep-alive
Keep-Alive: timeout=20
ETag: "5bcd60ad-8812"
Strict-Transport-Security: max-age=31557600; includeSubDomains
X-Frame-Options: SAMEORIGIN
X-Xss-Protection: 1
Referrer-Policy: same-origin
Accept-Ranges: bytes
```

I couldn't find out which version of `appstream-glib` you package, so I report it here. appstream-util from appstream-glib-0.6.13 validates the file.

Complete log:
```
% LC_ALL=C ~/AppImages/appimagetool-x86_64.AppImage -v install
appimagetool, continuous build (commit 225057b), build 1872 built on 2018-10-21 12:12:41 UTC
Desktop file: /home/tastytea/Projekte/whyblocked/build/install/de.tastytea.Whyblocked.desktop
/home/tastytea/Projekte/whyblocked/build/install/de.tastytea.Whyblocked.desktop: warning: key "Keywords" in group "Desktop Entry" is a reserved key for KDE
Name: Whyblocked
Icon: de.tastytea.Whyblocked
Exec: whyblocked-gui
Comment: Reminds you why you blocked someone
Type: Application
Categories: Utility;
/home/tastytea/Projekte/whyblocked/build/install/usr/bin/whyblocked used for determining architecture
x86_64
/home/tastytea/Projekte/whyblocked/build/install/usr/bin/whyblocked-gui used for determining architecture x86_64
Using architecture x86_64
App name for filename: Whyblocked
/home/tastytea/Projekte/whyblocked/build/install should be packaged as Whyblocked-x86_64.AppImage
AppStream upstream metadata found in usr/share/metainfo/de.tastytea.Whyblocked.appdata.xml
Trying to validate AppStream information with the appstreamcli tool
In case of issues, please refer to https://github.com/ximion/appstream
Validation was successful.
Trying to validate AppStream information with the appstream-util tool
In case of issues, please refer to https://github.com/hughsie/appstream-glib
/home/tastytea/Projekte/whyblocked/build/install/usr/share/metainfo/de.tastytea.Whyblocked.appdata.xml: FAILED:
? url-not-found : url not found [https://doc.schlomp.space/whyblocked/whyblocked_screenshot.png]
Validation of files failed
Failed to validate AppStream information with appstream-util
```

de.tastytea.Whyblocked.appdata.xml:
```XML

de.tastytea.Whyblocked.desktop
CC0-1.0
GPL-3.0
Whyblocked
Reminds you why you blocked someone
Erinnert dich, warum du jemanden blockiertest

Whyblocked allows you to store the reason why you blocked someone, along
with "receipts", URLs to the posts that led you to block them.

https://doc.schlomp.space/whyblocked/whyblocked_screenshot.png


https://schlomp.space/tastytea/whyblocked

```