Consider aes128gcm content encoding as an indicator for UnifiedPush

Without this a UnifiedPush/Web Push message with encryption would be
turned into an attachment. That in itself isn't pretty but can still
work, but it requires attachments to be enabled in the first place.

server/server.go

@@ -1025,7 +1025,8 @@ func (s *Server) parsePublishParams(r *http.Request, m *message) (cache bool, fi
 	}
 	template = readBoolParam(r, false, "x-template", "template", "tpl")
 	unifiedpush = readBoolParam(r, false, "x-unifiedpush", "unifiedpush", "up") // see GET too!
-	if unifiedpush {
+	contentEncoding := readParam(r, "content-encoding")
+	if unifiedpush || contentEncoding == "aes128gcm" {
 		firebase = false
 		unifiedpush = true
 	}