Bypass cache when requesting .prop file from WebDAV

This might not be necessary, but it's an attempt to troubleshoot
https://forums.zotero.org/discussion/63439/attachments-not-syncing,
which might be a caching issue.

chrome/content/zotero/xpcom/storage/webdav.js

@@ -1105,6 +1105,7 @@ Zotero.Sync.Storage.Mode.WebDAV.prototype = {
 				{
 					successCodes: [200, 300, 404],
 					requestObserver: xmlhttp => request.setChannel(xmlhttp.channel),
+					dontCache: true,
 					debug: true
 				}
 			);