fix cookie test key

[djih]

Cookiestorage has a little test to see if cookies are enabled in browser. The navigator.cookiesEnabled property yields false positives in IE, so we need to test directly by saving a value to cookie, fetching it, and verifying that the value matches.

Before we were using the current time as the test key/value. The problem with using a different key each time is that in rare cases the cookie.delete fails, and so whenever that user reloads the page, they accumulate a bunch of non-repeating cookie values, until their cookie hits max size.

The simple solution is just to use a constant key, with a changing value. If the delete fails, the next test will just overwrite the existing value, instead of creating new entries.

[paladin8]

lgtm