Fix memoizing of formatters when no locale is given.

[rikkert]

Currently the get format functions are not pure so they can't be memoized.
I noticed this during SSR while formatting dates and amounts without passing a locale, like so:

{$_.date(new Date(), { format: 'full' }).toUpperCase()}

and

{$_.number(10.99, { format: 'EUR' })}

This caches the internally created formatter for all further requests.
On other requests, in a different language, locale.set('xx-YY') the incorrect language would be used for formatting.

[kaisermann]

Oh, nice catch! Thanks 🎉. Will release it soon

[kaisermann]

Released in v2.3.1 😁