Fix UnifiedPush via Sunup not working

Thore Göbelrequested to merge
thgoebel/findmydeviceserver:webpush into master

Mozilla expects WebPush requests, so we need to look like one. Thanks for @julianfoad for finding this out.

Tested with both ntfy and Sunup.

Fixes findmydevice#319.

Edited by Thore Göbel

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