Tweaks to WebDAV preferences

Re http://forums.zotero.org/discussion/24528/report-1116370319-httphttps-syncing-to-webdav/
At least in Windows (perhaps because instantApply is off by default as opposed to Mac?) changing HTTPS to HTTP would not take effect until OK is pressed. So changing that value and trying to "Verify Server" would not make any difference.

Re http://forums.zotero.org/discussion/19255/30b1-typing-webdav-usernamepassword-very-slow-bug/
Because url and username were instantApply, `unverifyStorageServer`  was being triggered after every character.

Not sure if I'm overlooking something here. If unverifyStorageServer is asynchronous, maybe we need to setTimeout for applying the values to preferences? I assume the idea is to call `unverifyStorageServer` in order to disable sync before changing preferences.