[5.5] Fixed cache repository getMultiple implementation

[GrahamCampbell]

The $default param of Psr\SimpleCache\CacheInterface::getMultiple is meant to be the default value, and not an array of defaults, or something of a similar form

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For example, Symfony call getMultiple using new stdClass as the defaultm which crashes, because Laravel is trying to iterate over the parameter, instead of using it as the default.

Error Cannot use object of type stdClass as array 
    .../vendor/laravel/framework/src/Illuminate/Cache/Repository.php:131 Illuminate\Cache\Repository::getMultiple
    .../vendor/symfony/cache/Adapter/SimpleCacheAdapter.php:45 Symfony\Component\Cache\Adapter\SimpleCacheAdapter::doFetch
    .../vendor/symfony/cache/Adapter/AbstractAdapter.php:164 Symfony\Component\Cache\Adapter\AbstractAdapter::getItem
    .../vendor/php-http/cache-plugin/src/CachePlugin.php:143 Http\Client\Common\Plugin\CachePlugin::doHandleRequest

[driesvints]

@GrahamCampbell can you rebase with 5.5? Tests will pass then.

[GrahamCampbell]

Yeh, sure. :)