Enclose FreeBusyResponse in an array in areAvailable

[nuggex]

In the current master branc areAvailable fails with error PHP Warning: array_map(): Expected parameter 2 to be an array, object given in ../vendor/garethp/php-ews/src/CalendarAPI.php on line 314

This is due to the second argument to the array_map function not being an array.

[marclaporte]

@kambereBr Can you please evaluate this one? Tks

[kambereBr]

@nuggex the original error has changed due to massive changes in https://github.com/Garethp/php-ews/pull/265

Instead of the previous array_map error, it now shows:

Expected type 'object'. Found 'garethp\ews\API\Message\FreeBusyResponseType[]'.

This message error is currently appearing on lines 307 and 314 (src/CalendarAPI.php)

I suggest:

1. Rebase against current master to get latest changes from PR https://github.com/Garethp/php-ews/pull/265
2. Update your fix to address the new type mismatch error, not the old array_map issue

Thanks!