[5.8] Array cache forget should return false on missing key

[stevelacey]

The array cache driver currently returns true for all forget() calls. I think it should return false if there was no matching key, to better match the other drivers like memcached and redis.

I based this PR to 5.8 given anyone checking the return value of forget is probably expecting this behaviour? Not sure if this is BC incompatible enough to warrant pushing it to 6.0.

I am using the return value of forget to show whether a nonce is valid, so was expecting a true (and for the nonce to then be removed), and subsequent false's which works fine via the redis and memcached drivers, but not in the tests which use array.

[stevelacey]

Note, I used array_key_exists to ensure a true is returned when a cache key is set to null, which seems to be consistent with the other drivers

memcached, redis (and array, after this change):

>>> Cache::put('test', null)
=> true
>>> Cache::has('test')
=> false
>>> Cache::forget('test')
=> true
>>> Cache::forget('test')
=> false

array before this change:

>>> Cache::put('test', null)
=> true
>>> Cache::has('test')
=> false
>>> Cache::forget('test')
=> true
>>> Cache::forget('test')
=> true